Thermochimica

Acta

522 (2011) 110–

117

Contents

lists

available

at

ScienceDirect

Thermochimica

Acta

j o u r n a

l

h

o

m e

p a g e :

w w w . e l s e v i e r . c o m / l o c a t e / t c a

Crystal

polymorphism

of

poly(

l-lactic

acid)

and

its

influence

on

thermal

properties

Maria

Laura

Di

Lorenzo

∗

,

Mariacristina

Cocca,

Mario

Malinconico

Istituto

di

Chimica

e

Tecnologia

dei

Polimeri

(CNR),

c/o

Comprensorio

Olivetti,

Via

Campi

Flegrei,

34,

80078

Pozzuoli

(NA),Italy

a

r

t

i

c

l

e

i

n

f

o

Article

history:

Received

15

October

2010

Received

in

revised

form

16

December

2010

Accepted

22

December

2010

Available online 12 January 2011

Keywords:
Poly(

l-lactic

acid)

Polymorphism
Cold

crystallization

Rigid

amorphous

fraction

Thermal

analysis

a

b

s

t

r

a

c

t

The

influence

of

crystal

polymorphism

on

the

thermal

properties

of

poly(

l-lactic

acid)

(PLLA)

is

discussed

in

this

contribution.

Crystallization

of

PLLA

at

high

temperatures

yields

the

stable

␣

form,

whereas

at

low

temperatures

the

metastable

␣



modification

develops,

which

is

characterized

by

slightly

larger

lattice

dimensions

compared

to

the

␣

counterpart,

and

by

some

degree

of

conformational

disorder.

Quantitative

analysis

with

conventional

and

temperature-modulated

calorimetry

revealed

a

three-phase

structure

of

PLLA

composed

of

a

crystal

phase

and

two

amorphous

fractions

with

different

mobility,

for

all

the

analyzed

thermal

histories.

A

higher

coupling

of

the

amorphous

chain

segments

with

the

crystal

phase

was

found

in

the

presence

of

␣

crystals,

probably

due

to

the

slightly

larger

lattice

dimensions

and

the

looser

arrangements

of

PLLA

chains

in

the

␣



structure.

Some

peculiarities

in

the

thermal

behavior

were

rationalized,

like

an

unusual

frequency-dependence

of

the

reversing

apparent

heat

capacity

upon

the

solid–solid

transition

from

the

␣



to

the

␣

crystals.

Devitrification

of

the

rigid

amorphous

segments

seems

also

to

be

differently

affected

by

the

coupled

crystal

structure

for

the

two

analyzed

crystal

modifications

of

PLLA.

© 2011 Elsevier B.V. All rights reserved.

1.

Introduction

Poly(

l-lactic

acid)

(PLLA)

is

a

biodegradable

and

biocompati-

ble

polyester

that

can

be

produced

by

renewable

resources,

like

corn.

Being

non-toxic

to

human

body,

PLLA

is

used

in

biomedical

applications,

like

surgical

sutures,

bone

fixation

devices,

or

con-

trolled

drug

delivery.

Moreover,

the

good

mechanical

properties

and

easy

of

processability

make

PLLA

a

good

candidate

to

substitute

petroleum-based

polymers

in

selected

and

commodity

application,

with

the

added

value

of

biodegradability.

Similar

to

other

biodegradable

polyesters,

PLLA

displays

crystal

polymorphism,

as

three

main

different

crystal

modifications

can

develop,

named

␣,

␤,

and

␥

forms,

depending

on

preparation

con-

ditions.

The

␣

form

of

PLLA

grows

upon

melt

or

cold

crystallization,

as

well

as

from

solution.

The

␣

form

has

two

antiparallel

chains

in

a

left-handed

10

3

helical

conformation

(or

distorted

10

3

helix)

packed

in

an

orthorhombic

(or

pseudo-orthorhombic)

unit

cell

with

a

=

1.066

nm,

b

=

0.616

nm,

c

=

2.888

[1–3]

.

Hot-drawing

melt-spun

or

solution-spun

PLLA

fibers

to

a

high-draw

ratio

leads

to

the

␤

form.

An

orthorhombic

unit

cell

with

six

chains

in

the

3

1

helical

conformation,

with

axes

a

=

1.031

nm,

b

=

1.821

nm

and

c

=

0.900

nm

was

first

proposed

for

the

␤

modification

[4]

.

Similar

to

␣

crystals,

∗ Corresponding

author.

Tel.:

+39

081

867

5059;

fax:

+39

081

867

5230.

E-mail

address:

[e-mail]

(M.L.

Di

Lorenzo).

the

molecular

chains

of

the

␤

form

crystals

have

a

nearly

hexago-

nal

packing,

as

the

b/a

is

very

close

to

√

3

[3]

.

Puiggalí

et

al.

later

suggested

that

the

␤-form

rests

on

a

frustrated

packing

of

three

3

1

helix

chains

in

a

trigonal

unit

cell

with

parameters

a

=

b

=

1.052

nm,

c

=

0.880

nm,

˛

=

ˇ

=

90

◦

,



=

120

◦

,

with

a

space

group

P3

2

[5]

.

This

frustrated

structure

seems

to

be

formed

to

accommodate

the

ran-

dom

up-down

orientation

of

neighbor

chains

associated

with

the

rapid

crystallization

under

stretching

[5]

.

The

␥

form

is

obtained

via

epitaxial

crystallization

on

hex-

amethylbenzene

substrate.

It

is

characterized

by

two

antiparallel

helices

with

3

1

conformation

packed

in

an

orthorhombic

unit

cell

with

a

=

0.995

nm,

b

=

0.625

nm,

c

=

0.880

nm

[6]

.

The

a

(0.892

nm)

and

b

(0.886

nm)

axes

of

hexamethylbenzene

crystals

are

close

to

the

chain

axis

repeat

distance

of

the

␥

form

of

PLLA

in

the

3

1

heli-

cal

conformation

(0.880

nm).

This

matching

favors

the

epitaxial

growth

of

␥

form

crystals

of

PLLA

on

hexamethylbenzene

crystal

surface.

Besides

these

three

main

crystal

polymorphs,

a

disordered

mod-

ification

of

the

␣

form,

named

␣



form,

was

recently

proposed

for

PLLA.

The

WAXD

patterns

of

the

␣

and

␣



forms

of

PLLA

are

very

similar,

with

small

differences

seen

in

the

shift

to

higher

2



val-

ues

of

the

two

strongest

reflections,

assigned

to

the

(1

1

0)/(2

0

0)

and

(2

0

3)

planes,

and

in

the

appearance

of

a

weak

reflection

at

2



=

≈24.5

◦

in

the

␣



modification.

This

corresponds

to

a

similar

packing

of

the

two

polymorphs,

as,

analogous

to

the

␣

form,

the

PLLA

chains

in

the

␣



modification

have

a

10

3

helix

conformation

and

orthorhombic

(or

pseudo-orthorhombic)

unit

cell

[7–9]

.

The

0040-6031/$

–

see

front

matter ©

2011 Elsevier B.V. All rights reserved.

doi:

10.1016/j.tca.2010.12.027

M.L.

Di

Lorenzo

et

al.

/

Thermochimica

Acta

522 (2011) 110–

117

111

lattice

spacings

for

the

(1

1

0)/(2

0

0)

and

(2

0

3)

planes

of

␣



form

crystals

are

somewhat

larger

than

those

of

their

␣

counterparts,

indicating

that

the

␣



form

has

slightly

larger

lattice

dimensions

[7,10]

.

Upon

melt

or

cold

crystallization

conditions,

the

␣



form

is

known

to

grow

at

low

temperatures,

whereas

crystallization

at

high

temperatures

leads

to

formation

of

the

␣

modification.

The

exact

temperature

range

where

each

of

the

two

polymorphs

pre-

vails

depends

on

the

specific

PLLA

grade.

Upon

heating,

the

less

stable

␣



crystals

transform

to

the

␣

form,

which

results

in

the

appearance

of

multiple

endotherms

and

possible

exotherms

when

PLLA

is

analyzed

by

calorimetry

[8–10]

.

Crystal

polymorphism

is

known

to

have

a

large

influence

on

thermal

properties

of

semicrystalline

polymers.

The

variation

in

melting

behavior

caused

by

different

thermal

stability

of

the

crystal

modifications,

and

the

possible

interconversion

among

the

various

crystal

forms,

as

reported

for

PLLA,

are

the

most

commonly

ana-

lyzed

effects.

In

some

cases,

a

variation

in

the

crystal

modification

may

affect

not

only

the

crystal

phase,

but

also

the

thermal

prop-

erties

of

the

amorphous

segments.

This

is

the

case,

for

instance,

of

isotactic

poly(1-butene)

(PB-1),

as

the

spontaneous

transforma-

tion

of

the

metastable

form

II

to

the

more

stable

form

I

results

in

a

slight

increase

of

the

glass

transition

temperature

and

in

a

large

increase

of

the

rigid–amorphous

to

mobile–amorphous

ratio,

despite

an

unchanged

crystallinity

[11]

.

These

effects

are

due

not

only

to

a

shrink

of

the

crystals

associated

to

the

solid–solid

phase

transformation,

caused

by

a

much

higher

density

of

form

I

packing,

but

also

to

the

different

mobility

of

PB-1

chains

within

the

crystals,

as

the

large-amplitude

intramolecular

chain

motion

of

the

tetrag-

onal

form

II

makes

it

a

conformational

disordered

(condis)

crystal

[12]

.

Some

varied

degree

of

order

of

the

different

crystal

polymorphs

was

also

proposed

for

poly(

l-lactic

acid):

the

molecular

packing

within

the

unit

cell

of

␣



form

PLLA

is

looser

and

disordered,

with

larger

lattice

dimension

and

weaker

interchain

interaction

[7,9,10]

.

A

preliminary

analysis

by

Zhang

et

al.

[13]

suggested

that

the

chain

conformation

of

␣

and

␣



crystal

modifications

are

somewhat

dif-

ferent,

but

quantitative

results

have

not

been

reported

yet.

The

disorder

of

the

chains

within

the

␣



crystals

is

conformational,

which

makes

this

crystal

modification

a

mesophase

(condis

crys-

tal)

[14]

.

As

discussed

in

this

contribution,

the

varied

disorder

of

the

crystal

packing

in

PLLA

affects

the

thermal

properties

not

only

of

the

crystal

phase,

but

also

of

the

coupled

amorphous

portions.

Two

types

of

amorphous

fractions

are

usually

present

in

semicrystalline

polymers:

a

mobile

amorphous

phase

(MAF),

made

of

the

poly-

mer

chains

that

mobilize

at

the

glass

transition

temperature

(T

g

),

and

a

rigid

amorphous

fraction

(RAF),

made

of

the

polymer

chains

coupled

with

the

crystal

phase

that

usually

devitrify

at

higher

tem-

peratures

[15,16]

.

The

influence

of

crystal

polymorphism

on

the

relative

ratio

of

the

crystal

and

of

the

two

amorphous

fractions

is

also

analyzed

in

this

contribution.

As

the

thermal

properties

of

poly(lactic

acid)

are

highly

affected

by

the

stereochemistry

of

the

repeating

unit

[17]

,

a

polymer

with

a

very

high

amount

of

l-lactic

acid

was

used.

2.

Experimental

2.1.

Materials

and

sample

preparation

Poly(

l-lactic

acid)

(PLLA),

Biomer

L9000,

was

purchased

from

Biomer

Biopolyesters,

Germany.

Before

use

PLLA

was

dried

in

a

vac-

uum

oven

at

60

◦

C

for

24

h

to

avoid

hydrolysis

of

the

polymer

during

melt-processing.

After

drying,

the

PLLA

chips

were

compression-molded

with

a

Carver

Laboratory

Press

at

a

temperature

of

185

◦

C

for

4

min,

with-

out

any

applied

pressure,

to

allow

complete

melting.

After

this

period,

a

pressure

of

150

bar

was

applied

for

2

min.

Successively

the

press

plates,

equipped

with

cooling

coils,

were

quickly

cooled

to

room

temperature

by

cold

water.

The

as-prepared

PLLA

films

were

crystallized

in

oven

at

different

crystallization

temperatures

(T

c

=

85,

95,

105,

115,

125,

145,

165

◦

C)

for

18

h.

At

low

T

c

(85

◦

C)

the

crystallization

time

was

extended

to

66

h

because

of

the

slow

crystallization

rate,

as

discussed

below.

2.2.

Wide

angle

X-ray

analysis

The

crystalline

structure

of

PLLA

crystallized

at

different

T

c

was

investigated

by

wide-angle

X-ray

diffraction

analysis

(WAXS).

WAXS

investigations

were

carried

on

PLLA

films

by

means

of

a

Philips

(PW

1050

model)

powder

diffractometer

(Ni-filtered

CuK

␣

radiation)

equipped

with

a

rotative

sample

holder.

The

high

voltage

was

40

kV

and

the

tube

current

was

30

mA.

The

degree

of

crystallinity

(

w

C

)

of

PLLA

films

was

evaluated

according

to

the

Hermans–Weidinger

method,

as

w

C

is

given

by

the

ratio

between

the

diffraction

due

to

the

crystalline

phase

(I

c

)

and

the

total

diffraction

intensity

arising

from

both

the

amorphous

(I

a

)

and

crystal

parts

[18]

:

w

C

=

I

c

I

c

+

I

a

(1)

The

crystallinity

values

shown

below

are

averaged

from

seven

different

PLLA

films

for

each

T

c

.

2.3.

Calorimetry

The

thermal

properties

of

PLLA

films

were

measured

with

a

Perkin–Elmer

Pyris

Diamond

DSC,

equipped

with

Intracooler

II

as

cooling

system

and

with

a

Mettler

DSC

822

e

calorimeter

equipped

with

a

liquid-nitrogen

cooling

accessory.

Both

the

instruments

were

calibrated

in

temperature

with

high

purity

standards

(indium

and

cyclohexane)

and

in

energy

with

heat

of

fusion

of

indium.

Dry

nitrogen

was

used

as

purge

gas

at

a

rate

of

48

ml/min.

To

obtain

precise

heat

capacity

data,

each

measurement

was

accompanied

by

an

empty

pan

run,

and

a

calibration

run

with

sapphire

under

identical

conditions

[19]

.

All

the

measurements

were

repeated

at

least

three

times

to

improve

accuracy.

The

conventional

differential

scanning

calorimetry

(St-DSC)

analyses

were

conducted

with

the

Perkin–Elmer

Pyris

Diamond

DSC

at

the

scanning

rate

of

20

◦

C/min.

Temperature-modulated

calorimetry

(TMDSC)

at

the

underlying

heating

rate

of

2

◦

C

was

conducted

with

the

Perkin–Elmer

Pyris

Diamond

DSC

using

a

modulation

amplitude

of

0.4

◦

C

and

periods

of

temperature

oscilla-

tions

ranging

from

60

to

120

s.

Quasi-isothermal

TMDSC

data

were

gained

with

the

Mettler

DSC

822

e

calorimeter,

using

a

sawtooth

oscillation

with

a

temperature

amplitude

of

0.4

◦

C

and

a

modula-

tion

period

of

60

s

about

a

base

temperature

T

o

,

which

was

raised

stepwise

in

temperature

increments

of

5

◦

C

after

16

min

at

each

T

o

.

From

TMDSC

measurements

the

reversing

specific

heat

capac-

ity

was

obtained

from

the

ratio

of

the

amplitudes

of

modulated

heat

flow

rate

and

temperature,

both

approximated

with

Fourier

series

[20,21]

.

The

reversing

specific

heat

capacity

values

reported

in

this

contribution

were

obtained

from

the

first

harmonics

of

the

Fourier

series.

Similar

to

conventional

DSC

analyses,

each

TMDSC

measurement

was

accompanied

by

an

empty

pan

run,

and

a

cali-

bration

run

with

sapphire

under

identical

conditions

[19]

.

The

good

agreement

between

the

experimental

data

and

the

thermodynamic

heat

capacity

of

solid

and

liquid

PLLA

[22]

proves

that

the

modula-

tion

periods

used

are

long

enough

to

be

corrected

satisfactorily

by

the

calibration

with

sapphire.

112

M.L.

Di

Lorenzo

et

al.

/

Thermochimica

Acta

522 (2011) 110–

117

150

100

50

1.5

2.0

2.5

150

100

50

2

4

6

8

85°C 66 h

95°C 18 h

105°C 18 h

115°C 18 h

125°C 18 h

145°C 18 h

165°C 18 h

Temperature (°C)

c

p

[J/(K g)]

Fig.

1.

Specific

heat

capacity

of

PLLA

after

isothermal

cold

crystallization

at

the

indi-

cated

temperatures.

The

dashed

lines

are

the

solid

and

liquid

specific

heat

capacities

of

PLLA,

as

taken

from

Ref.

[22]

.

3.

Results

and

discussion

The

thermal

analysis

of

poly(

l-lactic

acid)

after

isothermal

cold

crystallization

at

various

temperatures

is

shown

in

Fig.

1

.

The

apparent

heat

capacity

(c

p

)

data

measured

upon

heating

at

the

con-

stant

linear

rate

of

20

◦

C/min

are

compared

to

thermodynamic

c

p

values

of

solid

and

liquid

PLLA,

as

taken

from

Ref.

[22]

.

The

multiple

melting

and

recrystallization

behavior

of

PLLA

is

largely

affected

by

the

thermal

history,

which

in

turn

determines

its

polymorphism

[8–10,23–25]

.

At

high

crystallization

temperatures

(T

c

≥

145

◦

C)

only

the

␣

form

is

present,

as

proven

by

the

WAXS

data

shown

below,

and

one

single

melting

peak

is

observed,

as

the

material

goes

on

fusion

directly

from

the

fully

ordered

crystal

to

the

melt,

without

changing

its

crystal

modification

[8,10,24]

.

PLLA

films

crys-

tallized

at

lower

temperatures,

where

either

␣

and

␣



forms

coexist,

or

␣



is

the

only

crystal

modification

present

in

the

film

before

the

DSC

scan,

display

multiple

thermal

events,

the

most

notable

ones

include

a

major

exotherm

after

partial

melting,

followed

by

a

large

endothermic

peak.

This

complex

melting

behavior

is

to

be

linked

to

metastability

of

␣



crystals,

that

convert

to

the

stable

␣

modification

during

heating

[10]

.

The

polymorphic

composition

of

PLLA

in

dependence

of

ther-

mal

history

was

determined

by

wide-angle

X-ray

diffraction.

Fig.

2

shows

the

WAXS

patterns

of

PLLA

after

crystallization

at

various

temperatures.

For

easier

comparison,

all

the

diffraction

patterns

were

normalized

using

the

strongest

(2

0

0)/(1

1

0)

reflection

inten-

sity

[24]

.

Indexing

of

the

observed

reflections

is

based

on

the

crystal

structure

reported

for

the

ordered

␣

modification

[26,27]

.

With

increasing

T

c

the

reflections

of

(2

0

0)/(1

1

0)

and

(2

0

3)

planes

shift

to

higher

2

,

together

with

an

increase

of

(0

1

0)

and

(0

1

5)

reflec-

tions

intensities,

evidenced

in

the

enlarged

WAXS

profiles

reported

in

Fig.

2

b.

Moreover,

small

diffraction

peaks

at

2



=

12.5

◦

,

20.8

◦

,

24.1

◦

,

and

25.1

◦

appear

at

high

T

c

,

which

are

assigned

to

the

reflec-

tions

of

(0

0

4)/(1

0

3),

(2

0

4),

(1

1

5),

(0

1

6),

and

(2

0

6)

planes

of

␣

crystals,

respectively,

while

they

are

absent

in

the

samples

crystal-

lized

at

T

c

≤

95

◦

C.

At

low

T

c

a

reflection

at

2



=

24.6

◦

,

characteristic

of

␣



crystals,

can

be

detected

[2,28]

.

These

results

suggest

that

at

T

c

≤

95

◦

C

the

analyzed

PLLA

grade

crystallizes

only

in

the

␣



form;

at

105

◦

C

≤

T

c

≤

125

◦

C

both

␣



and

␣

forms

coexist;

at

T

c

≥

145

◦

C

only

the

␣

modification

is

present,

which

is

in

good

agreement

with

the

available

literature

data

on

the

temperature-dependence

of

for-

mation

of

the

two

different

polymorphs

of

PLLA

[7,8,10,13,24,29]

.

The

WAXS

data

shown

in

Fig.

2

were

used

to

determine

the

crystal

fraction

of

PLLA

after

each

thermal

treatment.

This

proce-

dure

was

preferred

to

integration

of

the

DSC

melting

endotherms

because

of

the

complex

melting

behavior

of

PLLA,

especially

in

cases

30

25

20

15

10

Intensity (a.u.)

(203)

(110/200)

165°C

145°C

125°C

115°C

105°C

95°C

85°C

30

28

26

24

22

20

14

12

10

(115)

Intensity (a.u.)

αα

'

(018)

(207)

(206)

(016)

(015)

(204)

(010)

(004)/(103)

165°C

145°C

125°C

115°C

105°C

95°C

85°C

2

θ (°)

2

θ (°)

a

b

Fig.

2.

(a)

WAXS

profiles

of

PLLA

samples

crystallized

at

different

T

c

.

(b)

Enlarged

WAXS

profile

of

PLLA

samples

crystallized

at

different

T

c

.

where

the

initial

␣



crystals

transform

into

the

␣ structure

during

heating,

as

seen

in

Fig.

1

,

as

well

as

because

of

the

lack

of

precise

data

on

enthalpy

of

fusion

of

both

the

polymorphs

and

of

the

enthalpy

of

transition

from

the

metastable

to

the

stable

crystal

modification.

Besides

the

conventional

DSC

analyses

exhibited

in

Fig.

1

,

TMDSC

experiments

were

conducted

for

all

the

analyzed

crys-

tallization

temperatures.

Specific

examples

are

presented

for

two

selected

samples,

containing

only

one

of

the

two

analyzed

poly-

morphs,

to

illustrate

the

different

properties

of

the

two

crystal

modifications.

Fig.

3

a

reports

the

St-DSC

and

TMDSC

analyses

of

PLLA

after

isothermal

crystallization

at

85

◦

C

for

66

h.

On

the

same

plot,

the

St-DSC

analysis

of

PLLA

crystallized

at

85

◦

C

for

18

h

is

also

presented,

to

show

that

at

this

temperature

crystallization

of

PLLA

for

18

h

is

largely

incomplete.

This

is

confirmed

by

the

much

larger

heat

capacity

step

at

the

glass

transition,

that

indicates

a

higher

mobile

amorphous

fraction,

as

well

as

by

the

broad

exotherm

that

extends

from

about

85–90

◦

C

up

to

145

◦

C,

that

reveals

large

crys-

tallization

during

heating.

It

is

worth

to

note

that

in

the

poorly

crystallized

PLLA

the

glass

transition

of

the

MAF

is

located

at

lower

temperatures,

compared

to

the

polymer

maintained

at

T

c

for

much

longer

times,

which

reveals

the

marked

influence

of

the

semicrys-

talline

structure

on

the

amorphous

PLLA

chain

segments.

An

enlargement

of

the

PLLA

data

gained

after

crystallization

at

85

◦

C

for

66

h

is

illustrated

in

Fig.

3

b.

Below

the

glass

transition

region

and

above

completion

of

melting,

St-DSC

and

TMDSC

exper-

imental

data

well

agree

with

thermodynamic

c

p

of

solid

and

liquid

PLLA,

respectively.

The

specific

heat

capacity

of

PLLA,

measured

by

St-DSC,

starts

to

deviate

from

thermodynamic

c

p

of

solid

PLLA

at

around

60

◦

C,

in

correspondence

of

the

onset

of

the

glass

transition

M.L.

Di

Lorenzo

et

al.

/

Thermochimica

Acta

522 (2011) 110–

117

113

150

100

50

1.5

2.0

2.5

c

p

[J/(K g)]

Temperature (°C)

St-DSC 20°C/min
TMDSC p=60s
TMDSC p=90s
TMDSC p=120s
TMDSC Q-Iso

150

100

50

2

4

6

8

c

p

[J/(K g)]

Temperature (°C)

St-DSC 20°C/min
TMDSC p=60s
TMDSC p=90s
TMDSC p=120s
TMDSC Q-Iso
Tc=85°C 18 h

a

b

Fig.

3.

(a)

Specific

heat

capacity

of

PLLA

after

cold

crystallization

at

85

◦

C

for

66

h.

The

thick

black

solid

line

is

the

total

heat

capacity

by

St-DSC,

the

red,

green

and

blue

lines

are

the

reversing

specific

heat

capacity

measured

by

TMDSC

at

modulation

periods

p

=

60,

90,

120

s,

respectively,

the

yellow

circles

represent

the

reversing

heat

capacity

measured

in

quasi-isothermal

mode

of

modulation,

the

dashed

black

lines

are

the

solid

and

liquid

specific

heat

capacities,

as

taken

from

Ref.

[22]

.

The

St-DSC

data

of

PLLA

after

cold

crystallization

at

85

◦

C

for

18

h

are

also

shown

as

thin

black

solid

line.

(b)

Enlargement

of

the

plot

shown

in

(a)

in

the

area

of

changing

baseline

c

p

.

(For

interpretation

of

the

references

to

color

in

this

figure

legend,

the

reader

is

referred

to

the

web

version

of

this

article.)

of

the

mobile

amorphous

fraction.

In

the

temperature

region

of

the

glass

transition,

a

minor

frequency-dependence

of

the

reversing

heat

capacity

can

be

observed.

The

dynamic

T

g

,

i.e.

the

glass

transi-

tion

originating

from

temperature

modulation

and

obtainable

from

the

reversing

c

p

curve,

is

observed

at

temperatures

slightly

higher

than

the

devitrification

process

deriving

from

linear

heating

(ther-

mal

glass

transition).

This

can

be

explained

considering

that

the

frequencies

related

to

the

ordinary

linear

heating

rates

are

different

from

those

used

in

TMDSC

measurements,

the

latter

being

gener-

ally

higher

[30–32]

.

The

experimental

data

of

Fig.

3

were

used

to

determine

the

three-phase

composition

of

PLLA.

The

heat

capac-

ity

step

at

T

g

accounts

for

a

mobile

amorphous

phase

content

(

w

A

)

of

0.43.

The

crystal

fraction,

measured

by

WAXS,

is

w

C

=

0

.33.

The

rigid

amorphous

fraction

is

quantified

by

difference

using

Eq.

(2)

:

w

C

+

w

A

+

w

RA

=

1

(2)

which

yields

a

value

of

w

RA

=

0

.24

for

PLLA

after

cold

crystallization

at

85

◦

C

for

66

h.

A

notable

thermal

event

appears

in

Fig.

3

b

a

few

degrees

above

completion

of

the

glass

transition.

On

the

basis

of

St-DSC

data

only,

10

5

1.84

1.86

1.88

c

p

[J/(K g)]

Time (min)

Fig.

4.

Time

dependence

of

the

reversing

specific

heat

capacity

of

PLLA

during

quasi-

isothermal

TMDSC

analysis

at

100

◦

C.

this

thermal

event

may

be

interpreted

as

either

a

second

glass

tran-

sition,

followed

by

a

weak

and

broad

exotherm

that

extends

from

about

100

to

130–135

◦

C,

or

as

a

weak

and

broad

endotherm

cen-

tered

around

100

◦

C.

The

appearance

of

a

double

glass

transition

in

PLLA

was

reported

in

a

number

of

papers,

on

the

basis

of

St-

DSC

or

dynamical–mechanical

analyses

[33–35]

.

In

some

cases,

this

second

relaxation

was

ascribed

to

mobilization

of

the

rigid

amor-

phous

fraction.

From

above

the

glass

transition

temperature

up

to

about

100

◦

C

the

apparent

c

p

curve

measured

by

St-DSC

increases

beyond

the

c

p

level

that

corresponds

to

vitrified

rigid

amorphous

fraction.

This

may

be

connected

to

a

partial

devitrification

of

the

RAF,

as

seen

by

comparison

of

the

St-DSC

trace

with

the

base-

line

heat

capacities

drawn

in

Fig.

3

b

on

the

basis

of

the

two-phase

model,

that

accounts

for

the

crystal

phase

and

a

single

amorphous

fraction,

and

of

the

three-phase

model,

that

takes

into

account

the

crystal

and

two

amorphous

fractions

with

different

mobility,

respectively.

Quantitative

analysis

by

TMDSC

in

Fig.

3

b

shows

a

frequency-dependence

of

the

reversing

heat

capacity,

starting

from

80

◦

C,

which

may

indicate

some

reversing

exchange

of

latent

heat

in

this

temperature

range.

This

interpretation

may

be

not

unique,

since

during

devitrification

the

reversing

c

p

is

also

affected

by

the

periodicity

of

temperature

oscillation

[16]

,

as

also

seen

in

the

tem-

perature

range

of

the

glass

transition

of

the

MAF.

However,

the

two

processes

(fusion

and

devitrification)

have

different

response

to

small

oscillations

of

the

temperatures,

and

may

be

distinguished

by

quasi-isothermal

TMDSC

analysis,

which

usually

provides

differ-

ent

outputs

in

the

time

domain

when

a

polymer

is

analyzed

in

the

glass

transition

or

in

the

melting

range.

In

the

first

case

the

revers-

ing

c

p

remains

practically

constant

with

time,

whereas

a

slow

decay

is

observed

upon

reversing

melting

[16]

.

The

time-dependence

of

the

reversing

c

p

of

the

quasi-isothermal

TMDSC

analysis

of

Fig.

3

at

100

◦

C,

i.e.

at

the

peak

temperature

of

the

apparent

small

endotherm,

or

at

the

end

of

the

apparent

T

g

in

the

St-DSC

plot

of

Fig.

3

b,

is

exhibited

in

Fig.

4

.

The

slight

decrease

of

the

reversing

c

p

with

time

reveals

the

occurrence

of

some

reversing

melting,

and

that

the

frequency-dependence

of

the

TMDSC

curves

measured

at

the

underlying

heating

rate

of

2

◦

C/min

is

to

be

linked

to

latent

heat

exchanges

that

cause

an

increase

of

the

computed

reversing

c

p

beyond

the

reversible

c

p

values

[36–38]

.

The

thermal

event

under

analysis

can

therefore

be

linked

to

fusion

of

smaller

and/or

more

defective

crystals,

probably

grown

under

secondary

crystallization,

followed

by

crystallization

of

additional

chain

segments

above

100

◦

C.

No

quantitative

information

on

devitrification

of

the

RAF

cou-

pled

with

␣



crystals

can

be

derived

from

the

data

of

Fig.

3

,

due

to

the

overlapping

of

partial

melting

of

␣



crystals

and

transformation

114

M.L.

Di

Lorenzo

et

al.

/

Thermochimica

Acta

522 (2011) 110–

117

of

the

metastable

␣



structure

into

the

more

stable

␣

crystals.

The

quasi-isothermal

TMDSC

data

intersect

the

two-phase

baseline

at

135

◦

C,

but

the

temperature

at

which

the

reversing

heat

capacities

reaches

the

value

expected

for

full

devitrification

of

the

RAF

is

prob-

ably

affected

by

other

simultaneous

thermal

events,

which

may

increase

the

level

of

the

measured

reversing

c

p

.

Therefore,

it

is

likely

that

devitrification

of

the

RAF

reaches

completion

in

temperature

range

of

the

main

melting

endotherm

[39]

.

Another

noteworthy

feature

of

the

plots

shown

in

Fig.

3

a

is

the

unusual

frequency-dependence

of

the

reversing

heat

capacity

in

the

temperature

range

around

150

◦

C,

in

correspondence

of

the

exothermic

peak

visible

in

the

St-DSC

traces.

This

uncommon

trend

was

observed

for

all

analyzed

crystallization

temperatures,

where

some

amounts

of

␣



crystals

are

present

(85

≤

T

c

≤

125

◦

C).

As

men-

tioned

above,

in

correspondence

of

polymer

melting,

a

decrease

of

modulation

frequency

(or

an

increase

of

amplitude

of

temperature

oscillation),

usually

leads

to

a

higher

apparent

reversing

heat

capac-

ity,

because

a

decrease

in

the

frequency

of

modulation

permits

a

larger

percentage

of

crystalline

material

to

follow

the

modula-

tion

within

a

single

temperature

cycle

[11,35,40–46]

.

Similarly,

an

increase

in

modulation

amplitude

implies

that

a

higher

fraction

of

the

crystallites

that

is

involved

in

the

melting

process

is

added

to

the

reversing

signal.

This

kind

of

dependency

of

the

reversing

c

p

on

the

frequency

of

modulation

is

seen

in

the

data

of

Fig.

3

a,

except

around

150

◦

C,

where

the

data

gained

at

lower

modulation

period

display

a

higher

apparent

reversing

c

p

.

In

order

to

clarify

the

ori-

gin

of

this

unusual

trend,

the

raw

modulated

heat

flow

data

were

analyzed.

Fig.

5

reports

the

modulated

heat

flow

rate

of

PLLA

isothermally

crystallized

at

85

◦

C

for

66

h,

analyzed

by

TMDSC

at

the

underlying

heating

rate

of

2

◦

C/min

and

at

modulation

periods

of

60

and

120

s.

These

data

are

compared

in

Fig.

5

with

the

modulated

heat

flow

rate

with

the

same

modulation

parameters,

without

distortions

caused

by

the

occurrence

of

thermal

processes.

The

latter

curves

were

obtained

by

computer-simulation

from

the

experimental

raw

data

taken

above

completion

of

melting,

i.e.

in

absence

of

thermal

events,

using

the

procedure

detailed

in

Refs.

[38,47]

.

Comparison

of

experimental

and

simulated

heat

flow

rate

data

allows

to

deter-

mine

the

latent

heat

exchanged

during

each

oscillation

period.

Above

160

◦

C,

in

the

region

of

the

main

melting

peak,

large

dis-

tortions

in

the

experimental

curves

can

be

observed

in

both

their

endothermic

and

exothermic

parts,

and

the

effect

is

much

larger

at

higher

modulation

period.

As

a

result,

the

modulated

heat-flow-

rate

amplitude,

and

in

turn

the

reversing

c

p

,

increases

with

the

period

of

temperature

oscillation.

At

lower

temperatures,

around

150

◦

C,

the

modulated

heat-flow-rate

curves

are

deformed

to

a

lower

extent,

but

still

both

endothermic

and

exothermic

events

can

be

detected,

which

are

linked

to

partial

melting

and

to

the

ongoing

phase

transformation

from

the

metastable

␣



structure

to

the

stable

␣

form.

In

the

area

of

interest,

highlighted

by

the

arrow

in

Fig.

5

b,

in

the

experimental

curve

gained

at

p

=

120

s

endother-

mic

events

take

place

during

the

heating

segment

around

150

◦

C.

The

initial

increase

in

the

heat-flow-rate,

caused

by

the

switch

to

a

different

scanning

rate

overlapping

partial

melting,

is

followed

by

exothermal

effects,

as

revealed

by

comparison

with

the

sim-

ulated

data.

The

initial

increase

of

amplitude

of

modulated

heat

flow

rate

linked

to

latent

heat

release

is

followed

by

a

decrease

of

the

oscillation

amplitude,

as

the

experimental

modulated

heat

flow

rate

curve

falls

below

the

simulated

plot

before

the

switch

to

the

next

oscillation

segment.

Such

a

decrease

of

the

experi-

mental

data

below

the

level

corresponding

to

the

simulated

curve

is

not

seen

in

the

curve

gained

at

p

=

60

s,

shown

in

Fig.

5

a,

due

to

the

short

modulation

period.

Similarly,

in

the

preceding

half-

cycle

at

p

=

120

s,

the

exotherm

overlaps

endothermic

latent

heat

exchange,

and

again,

crosses

the

simulated

curve

before

the

end

of

the

modulation

half-period.

The

overall

result

is

that,

in

the

case

160

150

140

-5

0

5

10

Modulated

Φ

(W/g)

Temperature (°C)

p=60s (exp)
p=60s (calc)

160

150

140

-5

0

5

10

Modulated

Φ

(W/g)

Temperature (°C)

p=120s (exp)
p=120s (calc)

a

b

Fig.

5.

Experimental

and

simulated

heat-flow

rates

of

PLLA,

obtained

after

isother-

mal

crystallization

at

85

◦

C

for

66

h:

(a)

p

=

60

s

and

(b)

p

=

120

s.

of

long

period

of

oscillation,

p

=

120

s,

despite

the

larger

percentage

of

crystalline

material

that

follows

the

modulation

within

a

single

temperature

cycle,

the

neat

latent

heat

that

is

exchanged

in

each

modulation

cycle

(endothermic

minus

exothermic

heat)

is

lower

than

when

lower

periods

of

temperature

oscillation

are

used.

This

results

in

a

lower

amplitude

of

modulated

heat

flow

rate,

when

the

experimental

data

are

approximated

with

a

Fourier

series

in

each

modulation

cycle,

and

in

turn

in

a

lower

apparent

reversing

c

p

,

as

seen

in

Fig.

3

a

around

150

◦

C.

The

thermal

analysis

of

PLLA

after

isothermal

cold

crystalliza-

tion

at

145

◦

C

for

18

h

is

presented

in

Fig.

6

a,

with

an

enlargement

of

the

c

p

data

shown

in

Fig.

6

b.

As

revealed

by

the

WAXS

plots

of

Fig.

2

,

this

thermal

history

leads

to

development

of

the

␣ crystal

modifica-

tion

only,

and

a

single

major

melting

endotherm

appears

in

the

DSC

plots

of

Fig.

6

.

The

glass

transition

of

the

MAF

is

centered

at

64

◦

C,

a

few

degrees

below

the

T

g

of

the

polymer

crystallized

at

85

◦

C

for

66

h

(T

g

=

66

◦

C).

This

slight

decrease

of

the

T

g

of

PLLA

at

increas-

ing

crystallization

temperatures,

very

close

to

the

experimental

uncertainty,

is

in

agreement

with

literature

data

[48]

.

From

the

heat

capacity

step

at

T

g

a

mobile

amorphous

fraction

w

A

=

0

.31

is

measured,

which,

compared

to

the

w

A

=

0

.43

computed

after

crys-

tallization

at

85

◦

C

for

66

h,

indicates

that

crystallization

at

higher

temperatures

leads

to

a

reduction

of

the

MAF

content.

The

crys-

tal

fraction

measured

by

WAXS

after

crystallization

at

145

◦

C

is

w

C

=

0

.45,

which

leads

to

a

rigid

amorphous

content

w

RA

=

0

.24.

Above

completion

of

the

glass

transition,

the

St-DSC

and

the

TMDSC

data

of

Fig.

6

,

including

the

quasi-isothermal

analysis,

over-

M.L.

Di

Lorenzo

et

al.

/

Thermochimica

Acta

522 (2011) 110–

117

115

150

100

50

2

4

6

8

c

p

[J/(K g)]

Temperature (°C

)

St-DSC 20°C/min
TMDSC p=60s
TMDSC p=90s
TMDSC p=120s
TMDSC Q-Iso

150

100

50

1.5

2.0

2.5

c

p

[J/(K g)]

Temperature (°C

)

St-DSC 20°C/min
TMDSC p=60s
TMDSC p=90s
TMDSC p=120s
TMDSC Q-Iso

a

b

Fig.

6.

(a)

Specific

heat

capacity

of

PLLA

after

cold

crystallization

at

145

◦

C

for

18

h.

The

black

line

is

the

total

heat

capacity

by

St-DSC,

the

red,

green

and

blue

lines

are

the

reversing

specific

heat

capacity

measured

by

TMDSC

at

modulation

periods

p

=

60,

90,

120

s,

respectively,

the

yellow

circles

represent

the

reversing

heat

capacity

measured

in

quasi-isothermal

mode

of

modulation,

the

dashed

lines

are

the

solid

and

liquid

specific

heat

capacities,

as

taken

from

Ref.

[22]

.

(b)

Enlargement

of

the

plot

shown

in

(a)

in

the

area

of

changing

baseline

c

p

.

(For

interpretation

of

the

references

to

color

in

this

figure

legend,

the

reader

is

referred

to

the

web

version

of

this

article.)

lap

up

to

130

◦

C,

in

correspondence

of

the

onset

of

the

melting

endotherm,

which

starts

at

temperatures

slightly

lower

than

T

c

,

probably

due

to

some

residual

crystallization

during

cooling

to

room

temperature.

The

overlapping

of

reversing

and

total

c

p

data

reveals

that

negligible

latent

heat

exchanges

take

place

up

to

the

onset

of

melting

and

that

the

increase

of

the

experimental

c

p

values

up

to

the

beginning

of

melting

is

to

be

linked

to

devitrification

of

the

rigid

amorphous

segments

of

PLLA.

Unfortunately,

the

overlap-

ping

of

reversing

melting

to

the

c

p

increase

due

to

devitrification,

before

the

intersection

of

the

experimental

reversing

c

p

data

with

the

two-phase

baseline,

does

not

allow

to

estimate

the

exact

point

of

full

devitrification

of

the

RAF

of

PLLA

coupled

with

the

␣

crystals,

which

however

seems

to

attain

full

mobility

at

temperatures

close

to

the

onset

of

crystal

melting.

The

three-phase

composition

of

PLLA

after

isothermal

cold

crystallization

at

various

temperatures

is

illustrated

in

Fig.

7

.

Crys-

tallinity

increases

with

the

crystallization

temperature

in

the

whole

analyzed

range,

with

a

discontinuity

around

110–120

◦

C,

as

often

reported

in

the

literature

[49–53]

.

This

irregular

trend

is

to

be

linked

to

growth

of

PLLA

crystals

in

the

two

different

polymorphs

and

the

corresponding

varied

crystallization

kinetics.

The

mobile

amor-

phous

fraction

decreases

with

T

c

for

all

the

analyzed

crystallization

temperatures,

with

the

only

exception

of

a

slightly

lower

w

A

value

160

140

120

100

80

0.2

0.4

0.6

w

C

, w

A

, w

RA

T

c

(°C)

w

C

w

A

w

RA

Fig.

7.

Crystalline

(

w

C

),

mobile

amorphous

(

w

A

),

and

rigid

amorphous

(

w

RA

)

frac-

tions

of

PLLA

after

isothermal

cold

crystallization

at

various

T

c

.

measured

after

crystallization

at

85

◦

C

compared

to

crystallization

at

95

◦

C.

This

decrease

is

probably

related

to

the

extended

crystal-

lization

time

at

85

◦

C

(66

h)

compared

to

the

other

T

c

(18

h),

which

may

induce

a

larger

extent

of

secondary

crystallization,

with

possi-

ble

insertion

of

thin

lamellae

in

the

interlamellar

amorphous

layer

or

interlamellar

stacks

[54]

.

The

broad

and

weak

endotherm

in

the

DSC

data

of

Fig.

3

around

100

◦

C,

discussed

above,

confirms

the

exis-

tence

of

a

small

population

of

thin

and

defective

lamellae

with

very

poor

thermal

stability

in

PLLA

crystallized

at

85

◦

C

for

66

h.

It

is

interesting

to

note

that

a

decrease

of

the

crystallization

tem-

perature

from

95

to

85

◦

C

corresponds

to

a

considerable

increase

of

the

rigid

amorphous

fraction.

Upon

melt

crystallization,

it

has

been

often

reported

that

a

higher

rate

of

crystal

formation,

like

upon

crystallization

at

T

c

=

95

◦

C

compared

to

T

c

=

85

◦

C

[50]

,

leads

to

short

times

for

the

adjustment

of

the

crystals

into

the

locally

ener-

getically

most

favorable

states.

Internal

stresses

are

not

released

during

crystal

growth,

and

concentrate

at

the

interface

between

the

crystal

and

amorphous

phases,

resulting

in

a

large

rigid

amorphous

fraction

[16]

.

The

formation

of

secondary

crystals

upon

prolonged

crystallization

at

85

◦

C

may

also

be

linked

to

an

increased

coupling

between

the

amorphous

and

crystalline

areas:

rearrangements

of

the

amorphous

regions

localized

in

proximity

of

the

growing

sec-

ondary

lamellae

are

subjected

to

geometrical

restrictions,

in

which

the

melt

undergoes

larger

constraints,

with

consequent

forma-

tion

of

rigid

amorphous

phase.

This

may

confirm

the

hypothesis

often

appeared

in

the

literature,

of

a

connection

between

secondary

crystallization

and

vitrification

of

the

rigid

amorphous

segments

[40–46]

.

A

reduction

in

crystallization

temperature

corresponds

also

to

lower

chain

mobility,

especially

in

the

temperature

range

under

consideration,

that

is

very

close

to

the

glass

transition

of

the

mobile

amorphous

fraction,

which

may

complicate

rearrange-

ments

of

the

chains

at

the

crystal-amorphous

boundary,

leading

to

an

increased

fraction

of

amorphous

material

under

local

stress

at

the

crystal

surfaces.

The

w

RA

vs.

T

c

plot

of

Fig.

7

displays

a

maximum

at

125

◦

C.

At

high

temperatures

w

RA

decreases

with

T

c

,

as

commonly

reported

in

the

literature

for

a

number

of

semicrystalline

polymers

[16]

.

At

95

◦

C

≤

T

c

≤

125

◦

C

the

opposite

trend

can

be

observed.

An

increase

in

crystallization

temperature

in

this

range

corresponds

to

a

larger

fraction

of

␣ to

␣



crystals,

as

shown

in

Fig.

2

[55]

.

It

is

likely

that

crystallization

of

PLLA

into

the

ordered

␣

modification

leads

to

a

larger

coupling

of

the

amorphous

and

crystalline

chain

segments,

compared

to

the

conformationally

disordered

␣



arrangement.

A

similar

influence

of

the

varied

order

in

the

crystal

structure

was

116

M.L.

Di

Lorenzo

et

al.

/

Thermochimica

Acta

522 (2011) 110–

117

reported

for

isotactic

poly(1-butene)

[11]

.

In

PB-1

large-amplitude

motion

occurs

within

the

backbone

of

the

condis

crystals

of

form

II,

as

the

main

chains

adopt

locally

distributed

conformations

with

transitions

among

them

[56,12,57]

.

The

conformational

disordered

arrangement

in

PB-1

implies

segmental

motions

of

the

chains

within

the

lattice,

which

reduces

the

strain

on

the

amorphous

parts

of

the

molecules

coupled

to

the

crystal,

resulting

in

a

lesser

amount

of

RAF

in

semicrystalline

PB-1

with

crystals

of

form

II,

compared

to

the

most

stable

and

ordered

modification

I

[11]

.

Additionally,

as

mentioned

above,

the

unit

cell

of

the

␣



form

is

slightly

larger

than

that

of

the

␣

structure,

which

corresponds

to

a

looser

chain

arrange-

ment

within

the

␣



crystals

of

PLLA.

It

may

be

hypothesized

that,

similarly

to

PB-1,

tighter

arrangement

in

the

␣

modification

results

in

a

higher

strain

of

the

coupled

amorphous

chain

portions

at

the

crystal-amorphous

interface,

which

in

turn

causes

a

higher

fraction

of

rigid

amorphous

chain

segments

compared

to

the

␣



form

[11]

.

It

needs

to

be

underlined

that

the

three-phase

structure

of

PLLA

developed

upon

crystallization

at

various

temperatures

was

ana-

lyzed

after

cooling

to

room

temperatures.

Some

additional

crystal

growth

may

take

place

during

cooling

from

T

c

,

as

probed

in

Fig.

6

for

T

c

=

145

◦

C.

Vitrification

of

rigid

amorphous

portions

associated

to

crystals

growth

at

temperatures

below

T

c

cannot

be

excluded,

which

can

affect

the

data

reported

in

Fig.

7

.

Similarly,

for

a

num-

ber

of

crystallization

temperatures

the

crystallization

time

was

extended

beyond

completion

of

primary

crystallization,

as

detailed

in

Section

2

,

and

secondary

crystallization

may

occur

during

the

prolonged

exposure

at

T

c

[58]

.

Since

secondary

crystallization

may

be

linked

to

RAF

formation

[40–46]

,

the

prolonged

permanence

at

T

c

may

affect

the

three-phase

composition

of

PLLA.

4.

Conclusions

Quantitative

St-DSC

and

TMDSC

of

the

semicrystalline

PLLA

have

been

used

to

evaluate

the

thermodynamics

of

its

three-

phase

structure

which

is

globally

metastable.

The

contributions

from

vibrational

molecular

motion

to

the

heat

capacity

provide

the

baseline

for

the

solid

state

of

all

three

phases.

Combined

St-

DSC

and

TMDSC

analyses,

including

the

quasi-isothermal

mode

of

operation,

is

necessary

for

a

quantitative

assignment

of

the

ther-

mal

events,

which

is

not

accessible

by

conventional

DSC

alone

in

the

case

of

simultaneous

thermal

processes

that

may

lead

to

con-

troversial

interpretation

of

the

experimental

data.

An

example

is

demonstrated

in

the

case

of

the

low

temperature

melting

of

poor

and

defective

PLLA

␣



crystals,

a

thermal

event

that

may

be

con-

fused

with

a

partial

mobilization

of

amorphous

chain

portions

on

the

basis

of

qualitative

St-DSC

investigations

only.

The

complex

multiphase

structure

and

the

thermal

behavior

of

PLLA

are

largely

affected

by

crystal

polymorphism.

The

content

of

crystal

phase,

as

well

as

the

mobile

amorphous

and

rigid

amor-

phous

fractions,

depend

not

only

on

crystallization

kinetics

and

annealing,

but

also

on

the

crystal

modification,

with

a

higher

rigid

amorphous

content

in

the

presence

of

the

stable

␣

form.

This

results

not

only

from

the

higher

density

of

the

crystal

phase

in

the

␣

struc-

ture,

but

also

from

the

varied

mobility

of

the

crystals

coupled

with

the

RAF,

conformationally

disordered

in

␣



form

and

more

rigid

in

the

␣



form

I.

In

other

words,

in

PLLA

the

presence

of

a

specific

crys-

tal

modification

has

implications

not

only

on

the

arrangements

and

thermal

stability

of

the

chains

within

the

crystal

phase,

but

also

on

the

amorphous

chain

portions

coupled

with

the

more

or

less

ordered

polymorphs.

Acknowledgements

Financial

support

for

this

research

was

received

by

European

Commission

through

the

7FP

project

HORTIBIOPACK

(Grant

agree-

ment

n.

232551).

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Alemán,

B.

Lotz,

J.

Puiggalí,

Crystal

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4795–4801.

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Sasaki,

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Inoue,

Polymorphism

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Hoogsteen,

A.R.

Postema,

A.J.

Pennings,

G.

ten

Brinke,

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