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Structuring poly (lactic acid) film with excellent tensile toughness through extrusion blow molding
Journal Pre-proof Structuring poly (lactic acid) film with excellent tensile toughness through extrusion blow molding Cao Zengwen, Zhifeng Lu, Hongwei Pan, Junjia Bian, Lijing Han, Huiliang Zhang, Lisong Dong, Yuming Yang PII:
S0032-3861(19)31097-3
DOI:
https://doi.org/10.1016/j.polymer.2019.122091
Reference:
JPOL 122091
To appear in:
Polymer
Received Date: 4 November 2019 Revised Date:
4 December 2019
Accepted Date: 13 December 2019
Please cite this article as: Zengwen C, Lu Z, Pan H, Bian J, Han L, Zhang H, Dong L, Yang Y, Structuring poly (lactic acid) film with excellent tensile toughness through extrusion blow molding, Polymer (2020), doi: https://doi.org/10.1016/j.polymer.2019.122091. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
Structuring poly (lactic acid) film with excellent
2
tensile toughness through extrusion blow molding
3
Zengwen Cao, †‡ Zhifeng Lu, †‡ Hongwei Pan, † Junjia Bian, ** † Lijing Han, † Huiliang Zhang, †‡
4
Lisong Dong, †‡ and Yuming Yang *†‡
5
†
6
China
7
‡
8
ABSTRACT: We reveal a convenient and feasible processing technique to blow polylactide
9
(PLA) film with excellent tensile toughness, which was: PLA granules were isothermally
10
crystallized and then were blown at a temperature lower than the complete melting temperature.
11
The initial crystalline state of PLA granules was changed by adjusting isothermal crystallization
12
temperature, which was a key point in influencing to influence the condensed structures and
13
mechanical properties of films. The results showed that a lower annealing temperature was
14
beneficial for tension toughness of films. When the temperature was set at 90 °C, the elongation
15
at break of film reached 114% and 127% along transverse direction (TD) and machine direction
16
(MD), respectively. The mechanical performances of films were related with their condensed
17
structures. The residual crystals effectively induced tensile crystallization and mesophase during
18
blow molding. Crystals, acting as physical linked points, increased the stress transfer. Cohesive
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022,
University of Science and Technology of China, Hefei 230026, China
1
19
entanglement was an important factor causing PLA film embrittlement, which was suppressed by
20
mesophase.
21
Keywords: Poly (lactic acid), Cohesive entanglement, Mesophase
22
1. Introduction
23
Biodegradable polymers have received considerable development and attention recently due
24
to serious environmental pollution, caused by consumption of petroleum-based polymers. [1-4]
25
The pollution mainly is in the form of foam and film, and thus expanding the packaging
26
application of biodegradable polymer is very significant. Among several environmentally-
27
friendly materials, PLA is the most commendable polymer, which meets many requirements,
28
such as excellent physical property, gas barrier property as well as transparency. [2] However, the
29
high brittleness seriously limits the application of PLA film. So, it is still a challenge to produce
30
PLA film with excellent toughness.
31
Toughness is a complicated property, which can be defined as “impact strength/toughness”
32
or “tensile toughness”. The former means the ability to absorb sudden impact energy, and the
33
latter denotes the ability to absorb energy while being stretched during stretching. In terms of
34
packaging area, the tensile toughness was is used to judge whether the film is tough or not,
35
because it is always destructed by pulling apart. In order to improve the tensile toughness of PLA
36
film, many investigations have been addressed, including blending with nanofillers[5-8],
37
toughening
38
copolymerization.[19-22] However, there are still many difficulties among methods, including poor
39
interfacial incompatibility, migration aging, complicated operations, high cost and so on.
40
Comparatively speaking, structuring the single-polymer composite (SPC)[23-26] is a more
agents[9-14],
plasticizers[15,16],
uniaxial
or
biaxial
tension[17,18]
and
even
2
41
promising approach, which means the matrix and reinforcement coming from the same material.
42
As a typical polymer material, PLA has various structural units, which can be defined as matrix
43
and reinforcements, respectively. Due to the longer molecular chain structure, multiple motion
44
units coexist in PLA, which contains side groups, chain units, chain segments and whole chains
45
in the scale between around 1 nm and 10 nm. Besides, crystalline regions can be constructed by
46
chain segments, which are in the scale range of 0.1 to 1 mm. In general, the arrangement of
47
chains in crystals is closer than that in non-crystalline regions. And thus, the crystalline and non-
48
crystalline fields can be regarded as reinforcements and matrix of SPC PLA film, respectively.
49
As we know, the point linking structure and properties is molecular motion. Under the effect
50
of external forces, different scale motion units respond differently, which decide the end-use
51
properties of products. The molecular motions are affected by many factors, such as temperature,
52
molecular species, internal energy, cohesional entanglements, mesophase and other structural
53
units. The conception of cohesional entanglement is proposed by Renyuan Qian[27-29], which is
54
caused by the nematic alignment of two or three neighboring monomer units. The energy of
55
attractive cohesional interaction is very small, but the population of cohesional entanglement
56
may reach 3-5% monomer units of the chain. These entanglements and disentanglements are
57
easily formed, and they behave as clusters of monomer units with concentration fluctuations. The
58
cohesional entanglements, spacing along the whole chain, lock up the long-range cooperative
59
conformational changes, which impede the molecular motion and so make polymers prefer a
60
glassy state. Different from cohesional entanglements, the nature of mesophase is the first order
61
transition, where ∆Hmeso originates from the changes of intermolecular interaction rather than
62
conformational changes.[30] The phase is associated with the phenomenon of strain-hardening
63
during tension experiments.[31] Besides, the mesophase can be seen as an intermediate order
3
64
between amorphous and crystalline state.[32] During crystallization, the thin mesophase layers
65
form primitively, which still have mobility to some extent. And then, some stereo defects are
66
gradually expelled out, and the inner mesomorphic layers start thickening up to a critical value
67
where core regions crystallize into a block. At last, disordered surface regions of the block
68
rearrange, stabilize and develop into the final lamella. The crystalline regions may limit the
69
mobility of amorphous chains by tie molecules. In order to explain the restriction, the concept of
70
rigid amorphous fraction (RAF) is introduced, which is characterized as a nanoscale interfacial
71
region between crystalline and mobile amorphous fraction (MAF).[33.34] Up to now, the
72
conception of RAF is extended to the amorphous segments whose mobility is restrained because
73
of crystals, additive particles, orientation or other types of barriers. [35-37]
74
In fact, the boundaries among cohesional entanglement, mesophase and RAF are not clear.
75
For example, the thermal effects of cohesional entanglements and mesophase on polymer Tg are
76
nearly the same. The post-Tg endothermic peak that appears after sub-Tg annealing or stretching
77
with a proper drawing temperature, rate and strain, which attribute to cohesional entanglements
78
and mesophase respectively.[28-30,38] Moreover, Jianming Zhang reported that the mesophase
79
formed in melt-quenched PLA copolymer, which signified that annealing not only induced
80
cohesional entanglements but also mesomorphic phase in some cases.[39] Thus, the forming
81
condition may be same for cohesional entanglements and mesophase. Furthermore, Qian Ma
82
related mesophase and RAF in electrospun fibers.[40] In a word, the common microstructures
83
including cohesional entanglements, mesophase and RAF can transform reciprocally under the
84
same conditions, which depends on the types of polymer and processing methods. Nevertheless,
85
the relationships of among various scale structures and their effect on mechanical performances
86
have not been researched systematically.
4
87
A PLA chain is composed of L and D lactic acid stereoisomers. Due to the structural
88
difference of these sub-units (most PLA is made from co-monomers L-lactide, D-lactide and
89
meso-lactide, however the repeating unit in the polymer backbone is lactic acid), PLA can show
90
variation in the crystallization behaviors, such as low crystallization rate, multi-crystal types and
91
different lamella size or thickness based on the total D-lactic acid content of the PLA. The
92
crystalline morphology is connected with mechanical properties. For PLA granules with a bigger
93
size of crystals, more defects come into being at the interior of crystals or interfaces of crystals,
94
which decreases the mechanical properties of PLA. Similar reports have already been
95
published.[41] The existence of crystalline regions influences inevitably other scale structures,
96
such as cohesional entanglements, mesophase and RAF. Thus, the different responsive behaviors
97
to external forces may happen in particular temperature range and strain rate field, which
98
changes the performance of polymer.
99
In this work, the crystals with various sizes were introduced into PLA granules, which were
100
partially maintained during the blow molding process. Out of the annular die, PLA melt could
101
not quickly crystallize because of the low crystallization rate. These residual micro-crystals or
102
lamellae oriented and induced surrounding micro-structures formation. Differential scanning
103
calorimetry (DSC) and wide-angle x-ray diffraction (WAXD) were applied to quantitatively
104
characterize the change of structures. Shrinkage and Scanning Electronic Microscopy (SEM)
105
experiments were used to offer more evidences. Thus, the effect of initial crystalline state on
106
other scale structures was changed systematically during processing, and the relationship
107
between structures and properties was established in the context of a SPC PLA film.
108
2. Experiment section
109
2.1. Material
5
110
PLA, grade 2003D, purchased from Nature Works (USA) contained 4 % D lactic acid. And
111
the number and weight average molecular weight were 9.6×104 and 2.6 × 105 g/mol, respectively.
112
2.2. Sample Manufacturing
113
In order to remove moisture, PLA pellets were dried in an oven at 70 °C for 6 hours. A twin-
114
screw extruder (SHJ-20, The plastic mechanical engineering company of Lanzhoulantai, China)
115
was used to erase the thermal history of PLA pellets. The temperature range, from the hopper of
116
extruder to die, was set at 177, 183, 184, 185, 185, 185, 184, 183 and 181 °C, respectively. And
117
the rotational speed of screw (with a length-diameter ratio of 32) was set at 200 rpm. After melt
118
extrusion, the extrudates were cooled using cold water and then granulated. These extruded
119
pellets were dried and then annealed at different temperatures for 5 h. The isothermal
120
crystallization temperatures were set at 90, 110 and 120 °C, respectively. The crystalline pellets
121
of PLA were then processed into films employing a blown-film extrusion machine (Haake) with
122
a screw length: diameter ratio of 25: 1. The diameter of the annular die and die opening were 25
123
and 1 mm, respectively. The take-up ratio (TUR) and blow-up ratio (BUR) were set at 6.35 and
124
2.70, in order to control the film bubble stability and properties. For the simplicity of discussion,
125
blown films were denoted as PLA-1-2(3). In the code name, 1 represented condensed state of
126
PLA raw material including Crystalline (C) or amorphous (A). 2 expressed temperature regions
127
of blown-film extrusion, 155 °C or 180 °C and 3 was the isothermal crystallization temperature,
128
90, 110, 120 °C. The processing parameter including TUR, BUR and temperature of blow
129
molding were chosen after a series of preliminary experiments. (as seen in Fig. S1-3)
130
2.3. Polarized light optical microscopy (POM)
131
The optical images were recorded by a polarized light optical microscope (Leica DMLP
132
polarized microscope, Wetzlar, Germany) with a charge-coupled digital camera, which was
6
133
connected to hot stage (Linkam TM600). PLA raw material was sandwiched between two slices
134
of glass, and which was prepared by pressing at 180 °C. Following that, the specimens were
135
quenched to particular temperature (90, 100 and 120 °C, respectively) in order to observe the
136
evolution of crystals.
137
2.4. Thermal Properties
138
The thermal performances of polymer were characterized by differential scanning calorimetry
139
(DSC) experiments (TA Instruments DSC Q20, USA). In order to investigate the influence of
140
isothermal crystallization temperature on crystalline state, the PLA raw material weighing 5 to 8
141
mg was rapidly heated to 90, 110 and 120 °C for 5 h, respectively. Following that, the
142
temperature was quickly decreased to 0 °C and then increased to 185 °C with a rate of 10 °C/min.
143
DSC tests were also used to characterize the information about the thermal properties and micro-
144
structure of PLA film. The specimens were first heated to 185 °C at a rate of 10 °C/min and kept
145
for 3 min in order to eliminate the thermal history. After that, they were quickly cooled to 0 °C
146
and then a second upheated to 185 °C using the same heating rate. Nitrogen was pumped into
147
DSC cell at a gas press of 0.1 MPa for all measurements. The glass transition temperature (Tg),
148
cold crystallization temperature (Tcc), cold crystallization enthalpy (∆Hcc), melting temperature
149
(Tm) and heat of fusion (∆Hm) were determined by the thermal curves.
150
The classic three phase model of polymer contains crystalline phase (C), mobile amorphous
151
phase (MA) and rigid amorphous phase (RA). The crystallinity (XC) was calculated by ∆Hcc and
152
∆Hm using the following equation:
153
=
∆
∆ ∆
× 100%
(1)
7
154
where ∆ mo is the heat of fusion of PLA with spherulites of infinite size (93 J/g). The fraction
155
of MA (φMA) is obtained from the measured change of heat capacity (∆Cp) at Tg using the
156
following relation:
157
φMA = ∆Cp / ∆Cp0
158
where ∆Cp0 (∆Cp0 = 0.17 J/g/K) is the heat capacity step at Tg for 100% amorphous polymer.[42]
159
(2)
The content of RA (φRA) was estimated following equation (3):
160
φRA = 100 % - (φRA + XC)
161
2.5. Wide-Angle X-ray Diffraction (WAXD)
(3)
162
WAXD was carried out on the D8 Advance Bruker, and the wavelength (λ=1.54 Å) was
163
generated by a copper target. In our tests, diffraction angle (θ) was set from 5 to 60 o. The d-
164
spacing value was related to the λ and θ, which could be calculated by the equation: d=λ/2sinθ.
165
2.6. Mechanical Property Testing
166
Tensile behaviors of films were characterized employing a film tensile strength tester (XLW
167
(pc), China) based on ASTM D638-2003 at room temperature with a crosshead speed of 50
168
mm/min. The samples were previously cut from the center of the thin films using a special cutter
169
before testing. For each specimen, experiments were repeated five times and average value was
170
recorded to eliminate error.
171
2.7. Shrinkage
172
The shrinkage of films was measured by transferring specimens with fixed size into an air-
173
circulating oven for 12 h at 70 °C. As displayed in schematic diagram 1, the shrinkage ratios (η)
174
were calculated by the following equations:
8
Scheme 1. Two stages in the process of shrinkage: (a) original size of film;(b) size after thermal treatment 175
ηMD= (Linitial - Lfinal) / Linitial × 100%
176
ηTD= (Winitial - Wfinal) / Winitial × 100%
177
where Linitial and Lfinal are the length of film along MD before and after heat treatment, and Winitial
178
and Wfinal are along TD.
179
2.8. Scanning Electronic Microscopy (SEM)
180
The morphologies of films were observed by a field emission scanning electron microscopy
181
(XL30 ESEM FEG, FEI Co., Eindhoven, The Netherlands) with an accelerated voltage of 5 KV.
182
In order to better investigate the morphologies, films needed to be etched before transferring to
183
the SEM equipment. The film specimens were immersed in the 0.025 mol/L sodium hydroxide
184
mixture solution of water and methanol (Vwater: Vmethanol = 1:2) for 24 hours at room temperature.
185
The etched films were rinsed in distilled water with ultrasound irradiation for 2 hours. The films
186
were then dried in an oven for 12 hours at 50 °C and coated with a thin gold layer.
187
3. Results and discussion
188
3.1. The crystalline morphologies of PLA granules
9
189
The crystal morphologies of PLA granules at different isothermal temperature were displayed
190
in Fig. 1. When the annealing temperature was set at 90 °C, crystals with very small size
191
appeared after 60 min. Even though after 180 min, the average size of crystals was still small and
192
only a few maltese cross pattern were found. That attributed to the low segmental mobility at 90
193
°
C. Due to the inferior diffusion of chain segments, crystal growth was seriously restricted. And
194
so, many imperfect and small-size crystals developed. As the annealing temperature increased,
195
the segmental mobility also increased. When the temperature increased to 110 °C, the crystals
196
appeared after 5 min. The radius of spherulites was bigger, but the number of crystals than that
197
developed was fewer than at 90 °C. At 120 °C, the crystallization rate increased further, which
198
manifested as the increase crystal size, however, with fewer number appearing than at 110 °C in
199
the same time span. Thus, the crystallization morphologies of PLA granules were dependent on
200
the annealing temperature. Lower annealing temperature decreased segmental mobility, which
201
limited the arrangement of chain segments and so restrained crystallization rate and crystal
Fig. 1. Selected POM micrographs of PLA granules at different isothermal crystallization temperatures: (a) 90 °C; (b) 110 °C; (c)120 °C
10
202
Fig. 2. The first upheat of PLA granules annealed at 90, 110 and 120 °C
203
Fig. 3. DSC first heating curves of PLA films
204
Table 1. Thermal properties and relevant structure discrepancy of films
205 206 207 208
PLA-A-180
Tg (°C) Tcc(°C) Xfilm(%) Xtube(%) φMA(%) 57.00 112.21 1.30 57.94
PLA-C-155(90)
55.00
97.10
6.12
1.60
68.58
PLA-C-155(110)
55.40
97.81
4.14
2.54
71.88
PLA-C-155(120)
55.10
98.61
4.49
2.94
67.65
209
growth. As the annealing temperature increased, the segmental mobility improved and thus the
210
size of crystals increased. However, the higher temperature inhibited nucleating ability and
211
decreased the amount of crystals.
212
3.2. Thermal properties of PLA granules and films
11
213
The thermal properties of crystallized PLA granules at different temperature were further
214
characterized by DSC, as seen in Fig. 2. For the PLA granules isothermally crystallized at 90 °C,
215
two melting peaks appeared at 145.40 and 153.20 °C, which was result of two crystal types with
216
varying degree of defects. When the temperature was increased, imperfect crystals melted and
217
then recrystallized into complete crystals. The relatively complete crystals melted at higher
218
temperature. The double peaks corresponded to the crystals with different degree of perfection,
219
which was consistent with the picture of POM. When the isothermal crystallization temperature
220
increased to 110 °C, the double melting peaks disappeared, and was replaced with a single higher
221
melting peak at 151.47 °C. The melting peak further increased to 155.11 °C when annealing
222
temperature was set at 120 °C. The higher isothermal temperature was chosen, the stronger
223
mobility chain segments had. So, the chain arranged more closely and spherulites also grew more
224
perfectly. At the lower annealing temperature of 90 °C, there was limited chain mobility leading
225
to crystals with varying degree of perfection. That led to the observation of two melting points in
226
the DSC thermogram, a lower temperature of 145.40 °C and a higher temperature of 153.20 °C.
227
At the middle annealing temperature of 110 °C, there was more chain mobility leading to more
228
perfect crystals with an observed melting point of 151.47 °C. Finally, at the higher annealing
229
temperature of 120 °C, there was considerably more chain mobility leading to larger crystals
230
with the highest observed melting point of 155.11°C. It was the crystals with different
231
morphologies that induced various micro-structures during blow molding, and so the tension
232
toughness of PLA films improved.
233
The structure of polymer determined its properties via molecular movement, which could be
234
analyzed by thermal analysis. Fig. 3 displayed the DSC scans from the first upheat of the blown
235
films. For PLA-A-180, the Tg near 57.00 °C, Tcc at 112.21 and Tm were 147.11 °C, respectively.
12
236
For the blown films produced from crystalline PLA at 155 oC, regardless of the annealing
237
temperature, Tg and Tcc shifted to 55.00 °C and 98.00 °C, respectively. The lower Tg meant that
238
chain segments of PLA-C-155 were easier to move, in other words there was more free volume
239
for chain segments to move. Decreased Tcc illustrated that some structural units formed and
240
accelerated the cold crystallization process (the structural unit was mesophase, as discussed
241
below). Tube blank was the extrudate before blow molding and its crystallinity was also
242
measured by DSC, just as shown in Table 1 (concrete curves were shown in Fig. S4). The
243
crystallinity of film was bigger than tube, which implied tensile crystallization occurred during
244
blow molding.
245
As mentioned above, non-crystalline fraction of polymer could be divided into two groups
246
according to the mobility of chain segment, which were RA and MA. The proportion of MA
247
could be estimated by mathematical calculation, and the results were summarized in Table 1. The
248
increased proportion of MA corresponded to the elevated content of movable chain segments in
249
PLA-C-155, which meant that more plastic deformation might occur during tensile process.
250
3.3. The WAXD graphs of PLA films
251
In order to furthermore characterize the structure evolution of SPC PLA film during blow
252
molding, PLA-A-180 and PLA-C-155(110) were chosen to measure WAXD. Performing peak
253
fitting using PeakFit sofeware, the result indicated that PLA-A-180 was amorphous, just as Fig.
254
4a shown. For PLA-C-155(110) (Fig. 4b), mesophase and crystals appeared besides amorphous
255
phase, and the contents of them could be calculated by the area ratio of the diffraction peaks
256
from relevant phase and whole diffraction peaks. Combined DSC data, we believed that
257
mesophase and cohesive entanglement composed RAF, corresponding results were summarized
258
in Table 2. When extrusion temperature was set at 155 °C, extrudate of crystalline PLA still had
13
259
Fig. 4. Measured and fitting WAXD intensity profiles of PLA films:
260
(a)PLA-A-180; (b) PLA-C-155(110)
261
Table 2 Micro-structure contents of films
262 263 264 265
XMA
XRA
XC-DSC
Xmeso
XC-WAXD Xcohesive
(%)
(%)
(%)
(%)
(%)
(%)
PLA-A-180
57.94 40.76
1.30
0.00
-
40.76
PLA-C-155(90)
68.58 25.30
6.12
23.81
6.83
1.49
PLA-C-155(110) 71.88 23.98 PLA-C-155(120) 67.65 27.86
4.14 4.49
21.24 21.61
5.25 4.60
2.74 6.25
266
residual crystals (as shown in DSC data). These micro-crystals induced meosphase and oriented
267
crystallization during flow molding, which was proved by increasing mesophase and crystallinity.
268
It was mesophase that promoted the crystallization capacity of PLA-C-155, which could be
269
verified by decreased Tcc in DSC experiments. At the same time, cohesive entanglement was a
270
main reason of brittle fracture in PLA-A-180, which was effectively suppressed by mesophase
271
(decrease from 40.76 to 2.74 % for PLA-C-155(110)). (Chen, Y et al, 2018) In a word, residual crystals,
272
as a regulator, structured ductile PLA film by effectively adjusting micro-structures.
273
3.4. Mechanical properties of PLA films
274
Next, we characterized the mechanical properties of films and explored the relationship
275
between structure and properties. The amount of crystals decreased while the size increased as
14
276
increasing annealing temperature. Table 3 revealed that decreasing annealing temperature was
277
more favorable for increasing tension toughness of films, which displayed as bigger elongation at
278
break (ε). That attributed the smaller size of crystals to more effective depression of cohesional
279
entanglements during film blowing (The Xcohesive of PLA-C-155(90) was the smallest). The
280
stress-strain curves of PLA films were displayed in Fig. 5. As we could see, PLA-A-180 was
281
hard and brittle film, having high modulus (E) and low elongation at break. It was noticed that
282
PLA-C-155(90, 110 or 120) showed well-defined yield points, strain softening and strain
283
hardening behavior until fracture. Neck-in and plastic flow beyond the yield point could be
284
observed during overall tension process. The E was defined as a ratio of stress to strain in the
285
elastic limit, which was associated with the RA and crystalline phase. PLA-A-180 had higher
286
tensile modulus than other SPC PLA films because it had a higher proportion of cohesive
287
Fig. 5. Stress-strain curves of PLA films along different drawing directions.
288
Table 3. The mechanical properties of PLA films
Materials PLA-A-180 PLA-C-155(90)
E (GPa)
σTS (MPa)
ε (%)
TD/MD
TD/MD
TD/MD
3.41±0.11/4.42+0.34 36.67±4.32/57.52±3.51
2.63±0.83/3.52±1.64
2.42±0.06/4.09±0.24 44.54±3.65/50.22±4.37 113.87±5.73/126.76±5.38
PLA-C-155(110) 2.33±0.13/3.90±0.02 45.42±2.04/78.01±3.43
67.50±3.42/104.83±4.85
PLA-C-155(120) 2.53±0.22/4.11±0.25 33.92±1.73/41.05±2.96
36.42±2.63/99.08±3.46
15
289
entanglement (40.76%). Besides, SPC PLA films had better ability of plastic deformation, which
290
was proved by lower Tg, Xcohesive and higher XMA. And thus, the phenomenon of necking and
291
strain hardening were clearly observed in SPC PLA film during stretching. The increased tensile
292
strength (σTS) of SPC PLA films attributed to the tensile crystallization and strain hardening.
293
Specifically, for PLA-C-155(110), the elongation at break increased to 67.50 % and 104.83 %
294
along TD and MD, respectively. Meanwhile, the tensile strength increased from 36.67 MPa and
295
57.52 MPa to 45.42 MPa and 78.01 MPa along TD and MD, respectively. The detailed data were
296
summarized in table 3. It was the particular morphology structure caused by special processing
297
technology improved the mechanical properties of SPC PLA film.
298
3.5. Thermal shrinkage behaviors of PLA film
299
The improved tensile toughness of films attributed to the increased movability of chain
300
segments. And it was obvious to observe the anisotropy of tensile performance (as seen in Fig. 5),
301
which was usually ascribed to orientation of amorphous or crystalline phase. The PLA-C-155
302
(110) was chosen to investigate the influence of blow molding on orientation. The shrinkage (η)
303
was used to associate the tensile properties and oriented structure of film. Thermal stimulated
304
shrinkage, which was due to the increase of internal energy or stress in molecular level, mainly
305
occurred in oriented system. The orientation of chain segments was a manifestation of frozen
306
internal stress, which might relax and disorient once heating at proper temperature. Generally,
307
chain segments began to move when the temperature was higher than Tg. So, the orientation
308
Table 4. Shrinkage and tension properties of films
309 310
η along TD(%) η along MD(%) PLA-A-180
15.05
13.11
PLA-C-155(110)
21.00
32.34
311
16
Fig. 6. SEM micrographes of etched films: (a) PLA-C-155(110); (b) PLA-A-180; (3) PLA-A-155; (4) PLA-C-180(110) 312
degree of chain segments in non-crystalline region could be evaluated by shrinkage ratio at 70 oC.
313
Table 4 displayed that the shrinkage of PLA-C-155(110) was bigger than PLA-A-180, which
314
meant more oriented chain segments relaxed. Characterizing film shrinkage provided valuable
315
information exploring the evolution of condensed state during film-blowing. For PLA-C-
316
155(110), the increase of shrinkage ratio reflected the reinforced orientation of chain segments in
317
amorphous region and increase of frozen internal stress caused by residual crystals and lowered
318
melting temperature. Particularly, when film blowing was executed at a temperature near melting
319
point, lower movement ability of chains make PLA easily orient. When polymer molecules
320
exited the annular die, the pulling and inflating force tend to orient them along MD and TD,
321
respectively. And the orientation of chain segments along MD was bigger than TD as a result of
322
bigger pulling force. After thermal treatment of 70 oC, chain segments relaxed, and therefore P
323
LA-C-155(110) shrank.
324
3.6. The SEM images of film
325
The movability of MA was stronger than RA and crystals, which was more easily etched by a
326
water-methanol mixture solution. And SEM was used to vividly observe the orientation of film
327
(Fig. 6). The SEM micrograph of PLA-C-155(110) was chosen to display morphologies of SPC
17
Fig. 7. Schematic representation of constructing SCP PLA film using residual crystals and processing temperature of 155 °C 328
PLA film. As seen in Fig. 6a, PLA-C-155 displayed orientation structure in micro-scale.
329
However, PLA-A-180 showed nearly isotropy after etching (Fig. 6b). Combined with the film
330
shrinkage, the chain segments of PLA-A-180 oriented along TD and MD, which was discordant
331
with SEM picture. That was because that the orientation of PLA-A-180 was smaller and unstable,
332
which was destructed during etching process. In contrary, the orientation of PLA-C-155 was
333
effectively fixed by oriented crystals and tensile crystallization. So, the thread-like layers with
334
length as long as several micrometers were perfectly arranged along MD, interpenetrating some
335
amount of layers along TD. Neither PLA-A-155 nor PLA-C-180 displayed oriented
336
morphologies (Fig. 6c and d), indicating that crystalline region and lower processing temperature
337
(near melting point) were necessary to form the special structure.
338
3.7. Mechanism of toughness
339
Due to highly entangled nature of polymer chains, crystals or other factors, the movability of
340
some amorphous chain segments were restricted, namely RA. Based on the relationship between
341
structure and properties, we believed that polymer contains crystalline phase, MA and RA.
18
342
According to the concept of the RA, it mainly divided into mesophase and cohesive
343
entanglement. It was the cohesive entanglement network of polymer chains that leads to the
344
brittle of PLA film, which could be restrained by mesophase and orientation structure.[43] Fig. 7
345
illustrated the process of structuring mesophase and orientation structure. The crystal
346
morphologies of PLA granules were various at different isothermal crystallization temperature
347
range. Generally, lower crystallization temperature corresponded to the more quantity of crystals
348
with smaller size as well as more defects. The molten PLA still included some residual
349
microcrystals as the result of processing temperature lower the completely melting temperature.
350
When the melt exited the annular die, these residual crystals served as regulators to induce the
351
formation of mesophase and tensile crystallization under the effects of stretching due to the
352
pulling rollers and compressed air. The mesophase depressed the cohesive entanglement, which
353
was proved by WAXD and DSC. So, increased MA and mesophase endowed the PLA chains
354
with excellent plastic flow. Beyond that, the mobility of chains was lower at 155 °C than 180 °C,
355
which contributed to induced orientation structure. Meanwhile, the residual microcrystals and
356
tensile crystallization fixed the partial orientation, which were well coincided with the results of
357
shrinkage and SEM. The oriented but un-crystallized MA better transferred stress and have the
358
tendency to relax. According to Men and Wang, the tensile toughness of PLA film improved by
359
constructing a firmly amorphous network with increased chain movability.[44,45] Lower
360
crystallization temperature, the more residual microcrystals the polymer melt has, which more
361
effectively regulated condensed state and so improved the mechanical properties of film.
362
4. Conclusion
363
Although all polymers comprised of crystalline and non-crystalline phase, not all polymer
364
could be termed as SPC. It was due to that the crystals could not combine with orientation. After
19
365
isothermal crystallization and film molding near melting point, we preserved residual
366
microcrystals and then they, as regulators, effectively induced mesophase and tensile
367
crystallization. The mesophase depressed cohesive entanglements, which improved plastic
368
deformation of chains. Meanwhile, crystals fixed the orientation caused in the film inflating
369
process, and the oriented network effective transferred stress during stretching. The tensile
370
toughness of PLA film was greatly increased by structuring the special condensed state. It is an
371
excellent example to understand the relationship of structure and properties.
372
Appendix A. Supplementary data
373
The following is Supplementary data to this article
374
Author information
375
Corresponding Authors
376
* E-mail: [email protected],
377
** E-mail: [email protected].
378
Notes
379
There are no conflicts of interest to declare.
380
Acknowledgment
381
This work was financially supported by the Science and Technology Bureau of Jilin Province of
382
China (No. 20170204043GX) and the National Natural Science Foundation of China (No.
383
51503204)
384
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20
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24
Highlights: Residual microcrystals induced mesophase and tensile crystallization Mesophase depressed cohesive entanglements and improved plastic deformation of chains. The crystal-cross linked network effective transferred stress
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
There are no conflicts of interest to declare.
S0032-3861(19)31097-3
DOI:
https://doi.org/10.1016/j.polymer.2019.122091
Reference:
JPOL 122091
To appear in:
Polymer
Received Date: 4 November 2019 Revised Date:
4 December 2019
Accepted Date: 13 December 2019
Please cite this article as: Zengwen C, Lu Z, Pan H, Bian J, Han L, Zhang H, Dong L, Yang Y, Structuring poly (lactic acid) film with excellent tensile toughness through extrusion blow molding, Polymer (2020), doi: https://doi.org/10.1016/j.polymer.2019.122091. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
Structuring poly (lactic acid) film with excellent
2
tensile toughness through extrusion blow molding
3
Zengwen Cao, †‡ Zhifeng Lu, †‡ Hongwei Pan, † Junjia Bian, ** † Lijing Han, † Huiliang Zhang, †‡
4
Lisong Dong, †‡ and Yuming Yang *†‡
5
†
6
China
7
‡
8
ABSTRACT: We reveal a convenient and feasible processing technique to blow polylactide
9
(PLA) film with excellent tensile toughness, which was: PLA granules were isothermally
10
crystallized and then were blown at a temperature lower than the complete melting temperature.
11
The initial crystalline state of PLA granules was changed by adjusting isothermal crystallization
12
temperature, which was a key point in influencing to influence the condensed structures and
13
mechanical properties of films. The results showed that a lower annealing temperature was
14
beneficial for tension toughness of films. When the temperature was set at 90 °C, the elongation
15
at break of film reached 114% and 127% along transverse direction (TD) and machine direction
16
(MD), respectively. The mechanical performances of films were related with their condensed
17
structures. The residual crystals effectively induced tensile crystallization and mesophase during
18
blow molding. Crystals, acting as physical linked points, increased the stress transfer. Cohesive
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022,
University of Science and Technology of China, Hefei 230026, China
1
19
entanglement was an important factor causing PLA film embrittlement, which was suppressed by
20
mesophase.
21
Keywords: Poly (lactic acid), Cohesive entanglement, Mesophase
22
1. Introduction
23
Biodegradable polymers have received considerable development and attention recently due
24
to serious environmental pollution, caused by consumption of petroleum-based polymers. [1-4]
25
The pollution mainly is in the form of foam and film, and thus expanding the packaging
26
application of biodegradable polymer is very significant. Among several environmentally-
27
friendly materials, PLA is the most commendable polymer, which meets many requirements,
28
such as excellent physical property, gas barrier property as well as transparency. [2] However, the
29
high brittleness seriously limits the application of PLA film. So, it is still a challenge to produce
30
PLA film with excellent toughness.
31
Toughness is a complicated property, which can be defined as “impact strength/toughness”
32
or “tensile toughness”. The former means the ability to absorb sudden impact energy, and the
33
latter denotes the ability to absorb energy while being stretched during stretching. In terms of
34
packaging area, the tensile toughness was is used to judge whether the film is tough or not,
35
because it is always destructed by pulling apart. In order to improve the tensile toughness of PLA
36
film, many investigations have been addressed, including blending with nanofillers[5-8],
37
toughening
38
copolymerization.[19-22] However, there are still many difficulties among methods, including poor
39
interfacial incompatibility, migration aging, complicated operations, high cost and so on.
40
Comparatively speaking, structuring the single-polymer composite (SPC)[23-26] is a more
agents[9-14],
plasticizers[15,16],
uniaxial
or
biaxial
tension[17,18]
and
even
2
41
promising approach, which means the matrix and reinforcement coming from the same material.
42
As a typical polymer material, PLA has various structural units, which can be defined as matrix
43
and reinforcements, respectively. Due to the longer molecular chain structure, multiple motion
44
units coexist in PLA, which contains side groups, chain units, chain segments and whole chains
45
in the scale between around 1 nm and 10 nm. Besides, crystalline regions can be constructed by
46
chain segments, which are in the scale range of 0.1 to 1 mm. In general, the arrangement of
47
chains in crystals is closer than that in non-crystalline regions. And thus, the crystalline and non-
48
crystalline fields can be regarded as reinforcements and matrix of SPC PLA film, respectively.
49
As we know, the point linking structure and properties is molecular motion. Under the effect
50
of external forces, different scale motion units respond differently, which decide the end-use
51
properties of products. The molecular motions are affected by many factors, such as temperature,
52
molecular species, internal energy, cohesional entanglements, mesophase and other structural
53
units. The conception of cohesional entanglement is proposed by Renyuan Qian[27-29], which is
54
caused by the nematic alignment of two or three neighboring monomer units. The energy of
55
attractive cohesional interaction is very small, but the population of cohesional entanglement
56
may reach 3-5% monomer units of the chain. These entanglements and disentanglements are
57
easily formed, and they behave as clusters of monomer units with concentration fluctuations. The
58
cohesional entanglements, spacing along the whole chain, lock up the long-range cooperative
59
conformational changes, which impede the molecular motion and so make polymers prefer a
60
glassy state. Different from cohesional entanglements, the nature of mesophase is the first order
61
transition, where ∆Hmeso originates from the changes of intermolecular interaction rather than
62
conformational changes.[30] The phase is associated with the phenomenon of strain-hardening
63
during tension experiments.[31] Besides, the mesophase can be seen as an intermediate order
3
64
between amorphous and crystalline state.[32] During crystallization, the thin mesophase layers
65
form primitively, which still have mobility to some extent. And then, some stereo defects are
66
gradually expelled out, and the inner mesomorphic layers start thickening up to a critical value
67
where core regions crystallize into a block. At last, disordered surface regions of the block
68
rearrange, stabilize and develop into the final lamella. The crystalline regions may limit the
69
mobility of amorphous chains by tie molecules. In order to explain the restriction, the concept of
70
rigid amorphous fraction (RAF) is introduced, which is characterized as a nanoscale interfacial
71
region between crystalline and mobile amorphous fraction (MAF).[33.34] Up to now, the
72
conception of RAF is extended to the amorphous segments whose mobility is restrained because
73
of crystals, additive particles, orientation or other types of barriers. [35-37]
74
In fact, the boundaries among cohesional entanglement, mesophase and RAF are not clear.
75
For example, the thermal effects of cohesional entanglements and mesophase on polymer Tg are
76
nearly the same. The post-Tg endothermic peak that appears after sub-Tg annealing or stretching
77
with a proper drawing temperature, rate and strain, which attribute to cohesional entanglements
78
and mesophase respectively.[28-30,38] Moreover, Jianming Zhang reported that the mesophase
79
formed in melt-quenched PLA copolymer, which signified that annealing not only induced
80
cohesional entanglements but also mesomorphic phase in some cases.[39] Thus, the forming
81
condition may be same for cohesional entanglements and mesophase. Furthermore, Qian Ma
82
related mesophase and RAF in electrospun fibers.[40] In a word, the common microstructures
83
including cohesional entanglements, mesophase and RAF can transform reciprocally under the
84
same conditions, which depends on the types of polymer and processing methods. Nevertheless,
85
the relationships of among various scale structures and their effect on mechanical performances
86
have not been researched systematically.
4
87
A PLA chain is composed of L and D lactic acid stereoisomers. Due to the structural
88
difference of these sub-units (most PLA is made from co-monomers L-lactide, D-lactide and
89
meso-lactide, however the repeating unit in the polymer backbone is lactic acid), PLA can show
90
variation in the crystallization behaviors, such as low crystallization rate, multi-crystal types and
91
different lamella size or thickness based on the total D-lactic acid content of the PLA. The
92
crystalline morphology is connected with mechanical properties. For PLA granules with a bigger
93
size of crystals, more defects come into being at the interior of crystals or interfaces of crystals,
94
which decreases the mechanical properties of PLA. Similar reports have already been
95
published.[41] The existence of crystalline regions influences inevitably other scale structures,
96
such as cohesional entanglements, mesophase and RAF. Thus, the different responsive behaviors
97
to external forces may happen in particular temperature range and strain rate field, which
98
changes the performance of polymer.
99
In this work, the crystals with various sizes were introduced into PLA granules, which were
100
partially maintained during the blow molding process. Out of the annular die, PLA melt could
101
not quickly crystallize because of the low crystallization rate. These residual micro-crystals or
102
lamellae oriented and induced surrounding micro-structures formation. Differential scanning
103
calorimetry (DSC) and wide-angle x-ray diffraction (WAXD) were applied to quantitatively
104
characterize the change of structures. Shrinkage and Scanning Electronic Microscopy (SEM)
105
experiments were used to offer more evidences. Thus, the effect of initial crystalline state on
106
other scale structures was changed systematically during processing, and the relationship
107
between structures and properties was established in the context of a SPC PLA film.
108
2. Experiment section
109
2.1. Material
5
110
PLA, grade 2003D, purchased from Nature Works (USA) contained 4 % D lactic acid. And
111
the number and weight average molecular weight were 9.6×104 and 2.6 × 105 g/mol, respectively.
112
2.2. Sample Manufacturing
113
In order to remove moisture, PLA pellets were dried in an oven at 70 °C for 6 hours. A twin-
114
screw extruder (SHJ-20, The plastic mechanical engineering company of Lanzhoulantai, China)
115
was used to erase the thermal history of PLA pellets. The temperature range, from the hopper of
116
extruder to die, was set at 177, 183, 184, 185, 185, 185, 184, 183 and 181 °C, respectively. And
117
the rotational speed of screw (with a length-diameter ratio of 32) was set at 200 rpm. After melt
118
extrusion, the extrudates were cooled using cold water and then granulated. These extruded
119
pellets were dried and then annealed at different temperatures for 5 h. The isothermal
120
crystallization temperatures were set at 90, 110 and 120 °C, respectively. The crystalline pellets
121
of PLA were then processed into films employing a blown-film extrusion machine (Haake) with
122
a screw length: diameter ratio of 25: 1. The diameter of the annular die and die opening were 25
123
and 1 mm, respectively. The take-up ratio (TUR) and blow-up ratio (BUR) were set at 6.35 and
124
2.70, in order to control the film bubble stability and properties. For the simplicity of discussion,
125
blown films were denoted as PLA-1-2(3). In the code name, 1 represented condensed state of
126
PLA raw material including Crystalline (C) or amorphous (A). 2 expressed temperature regions
127
of blown-film extrusion, 155 °C or 180 °C and 3 was the isothermal crystallization temperature,
128
90, 110, 120 °C. The processing parameter including TUR, BUR and temperature of blow
129
molding were chosen after a series of preliminary experiments. (as seen in Fig. S1-3)
130
2.3. Polarized light optical microscopy (POM)
131
The optical images were recorded by a polarized light optical microscope (Leica DMLP
132
polarized microscope, Wetzlar, Germany) with a charge-coupled digital camera, which was
6
133
connected to hot stage (Linkam TM600). PLA raw material was sandwiched between two slices
134
of glass, and which was prepared by pressing at 180 °C. Following that, the specimens were
135
quenched to particular temperature (90, 100 and 120 °C, respectively) in order to observe the
136
evolution of crystals.
137
2.4. Thermal Properties
138
The thermal performances of polymer were characterized by differential scanning calorimetry
139
(DSC) experiments (TA Instruments DSC Q20, USA). In order to investigate the influence of
140
isothermal crystallization temperature on crystalline state, the PLA raw material weighing 5 to 8
141
mg was rapidly heated to 90, 110 and 120 °C for 5 h, respectively. Following that, the
142
temperature was quickly decreased to 0 °C and then increased to 185 °C with a rate of 10 °C/min.
143
DSC tests were also used to characterize the information about the thermal properties and micro-
144
structure of PLA film. The specimens were first heated to 185 °C at a rate of 10 °C/min and kept
145
for 3 min in order to eliminate the thermal history. After that, they were quickly cooled to 0 °C
146
and then a second upheated to 185 °C using the same heating rate. Nitrogen was pumped into
147
DSC cell at a gas press of 0.1 MPa for all measurements. The glass transition temperature (Tg),
148
cold crystallization temperature (Tcc), cold crystallization enthalpy (∆Hcc), melting temperature
149
(Tm) and heat of fusion (∆Hm) were determined by the thermal curves.
150
The classic three phase model of polymer contains crystalline phase (C), mobile amorphous
151
phase (MA) and rigid amorphous phase (RA). The crystallinity (XC) was calculated by ∆Hcc and
152
∆Hm using the following equation:
153
=
∆
∆ ∆
× 100%
(1)
7
154
where ∆ mo is the heat of fusion of PLA with spherulites of infinite size (93 J/g). The fraction
155
of MA (φMA) is obtained from the measured change of heat capacity (∆Cp) at Tg using the
156
following relation:
157
φMA = ∆Cp / ∆Cp0
158
where ∆Cp0 (∆Cp0 = 0.17 J/g/K) is the heat capacity step at Tg for 100% amorphous polymer.[42]
159
(2)
The content of RA (φRA) was estimated following equation (3):
160
φRA = 100 % - (φRA + XC)
161
2.5. Wide-Angle X-ray Diffraction (WAXD)
(3)
162
WAXD was carried out on the D8 Advance Bruker, and the wavelength (λ=1.54 Å) was
163
generated by a copper target. In our tests, diffraction angle (θ) was set from 5 to 60 o. The d-
164
spacing value was related to the λ and θ, which could be calculated by the equation: d=λ/2sinθ.
165
2.6. Mechanical Property Testing
166
Tensile behaviors of films were characterized employing a film tensile strength tester (XLW
167
(pc), China) based on ASTM D638-2003 at room temperature with a crosshead speed of 50
168
mm/min. The samples were previously cut from the center of the thin films using a special cutter
169
before testing. For each specimen, experiments were repeated five times and average value was
170
recorded to eliminate error.
171
2.7. Shrinkage
172
The shrinkage of films was measured by transferring specimens with fixed size into an air-
173
circulating oven for 12 h at 70 °C. As displayed in schematic diagram 1, the shrinkage ratios (η)
174
were calculated by the following equations:
8
Scheme 1. Two stages in the process of shrinkage: (a) original size of film;(b) size after thermal treatment 175
ηMD= (Linitial - Lfinal) / Linitial × 100%
176
ηTD= (Winitial - Wfinal) / Winitial × 100%
177
where Linitial and Lfinal are the length of film along MD before and after heat treatment, and Winitial
178
and Wfinal are along TD.
179
2.8. Scanning Electronic Microscopy (SEM)
180
The morphologies of films were observed by a field emission scanning electron microscopy
181
(XL30 ESEM FEG, FEI Co., Eindhoven, The Netherlands) with an accelerated voltage of 5 KV.
182
In order to better investigate the morphologies, films needed to be etched before transferring to
183
the SEM equipment. The film specimens were immersed in the 0.025 mol/L sodium hydroxide
184
mixture solution of water and methanol (Vwater: Vmethanol = 1:2) for 24 hours at room temperature.
185
The etched films were rinsed in distilled water with ultrasound irradiation for 2 hours. The films
186
were then dried in an oven for 12 hours at 50 °C and coated with a thin gold layer.
187
3. Results and discussion
188
3.1. The crystalline morphologies of PLA granules
9
189
The crystal morphologies of PLA granules at different isothermal temperature were displayed
190
in Fig. 1. When the annealing temperature was set at 90 °C, crystals with very small size
191
appeared after 60 min. Even though after 180 min, the average size of crystals was still small and
192
only a few maltese cross pattern were found. That attributed to the low segmental mobility at 90
193
°
C. Due to the inferior diffusion of chain segments, crystal growth was seriously restricted. And
194
so, many imperfect and small-size crystals developed. As the annealing temperature increased,
195
the segmental mobility also increased. When the temperature increased to 110 °C, the crystals
196
appeared after 5 min. The radius of spherulites was bigger, but the number of crystals than that
197
developed was fewer than at 90 °C. At 120 °C, the crystallization rate increased further, which
198
manifested as the increase crystal size, however, with fewer number appearing than at 110 °C in
199
the same time span. Thus, the crystallization morphologies of PLA granules were dependent on
200
the annealing temperature. Lower annealing temperature decreased segmental mobility, which
201
limited the arrangement of chain segments and so restrained crystallization rate and crystal
Fig. 1. Selected POM micrographs of PLA granules at different isothermal crystallization temperatures: (a) 90 °C; (b) 110 °C; (c)120 °C
10
202
Fig. 2. The first upheat of PLA granules annealed at 90, 110 and 120 °C
203
Fig. 3. DSC first heating curves of PLA films
204
Table 1. Thermal properties and relevant structure discrepancy of films
205 206 207 208
PLA-A-180
Tg (°C) Tcc(°C) Xfilm(%) Xtube(%) φMA(%) 57.00 112.21 1.30 57.94
PLA-C-155(90)
55.00
97.10
6.12
1.60
68.58
PLA-C-155(110)
55.40
97.81
4.14
2.54
71.88
PLA-C-155(120)
55.10
98.61
4.49
2.94
67.65
209
growth. As the annealing temperature increased, the segmental mobility improved and thus the
210
size of crystals increased. However, the higher temperature inhibited nucleating ability and
211
decreased the amount of crystals.
212
3.2. Thermal properties of PLA granules and films
11
213
The thermal properties of crystallized PLA granules at different temperature were further
214
characterized by DSC, as seen in Fig. 2. For the PLA granules isothermally crystallized at 90 °C,
215
two melting peaks appeared at 145.40 and 153.20 °C, which was result of two crystal types with
216
varying degree of defects. When the temperature was increased, imperfect crystals melted and
217
then recrystallized into complete crystals. The relatively complete crystals melted at higher
218
temperature. The double peaks corresponded to the crystals with different degree of perfection,
219
which was consistent with the picture of POM. When the isothermal crystallization temperature
220
increased to 110 °C, the double melting peaks disappeared, and was replaced with a single higher
221
melting peak at 151.47 °C. The melting peak further increased to 155.11 °C when annealing
222
temperature was set at 120 °C. The higher isothermal temperature was chosen, the stronger
223
mobility chain segments had. So, the chain arranged more closely and spherulites also grew more
224
perfectly. At the lower annealing temperature of 90 °C, there was limited chain mobility leading
225
to crystals with varying degree of perfection. That led to the observation of two melting points in
226
the DSC thermogram, a lower temperature of 145.40 °C and a higher temperature of 153.20 °C.
227
At the middle annealing temperature of 110 °C, there was more chain mobility leading to more
228
perfect crystals with an observed melting point of 151.47 °C. Finally, at the higher annealing
229
temperature of 120 °C, there was considerably more chain mobility leading to larger crystals
230
with the highest observed melting point of 155.11°C. It was the crystals with different
231
morphologies that induced various micro-structures during blow molding, and so the tension
232
toughness of PLA films improved.
233
The structure of polymer determined its properties via molecular movement, which could be
234
analyzed by thermal analysis. Fig. 3 displayed the DSC scans from the first upheat of the blown
235
films. For PLA-A-180, the Tg near 57.00 °C, Tcc at 112.21 and Tm were 147.11 °C, respectively.
12
236
For the blown films produced from crystalline PLA at 155 oC, regardless of the annealing
237
temperature, Tg and Tcc shifted to 55.00 °C and 98.00 °C, respectively. The lower Tg meant that
238
chain segments of PLA-C-155 were easier to move, in other words there was more free volume
239
for chain segments to move. Decreased Tcc illustrated that some structural units formed and
240
accelerated the cold crystallization process (the structural unit was mesophase, as discussed
241
below). Tube blank was the extrudate before blow molding and its crystallinity was also
242
measured by DSC, just as shown in Table 1 (concrete curves were shown in Fig. S4). The
243
crystallinity of film was bigger than tube, which implied tensile crystallization occurred during
244
blow molding.
245
As mentioned above, non-crystalline fraction of polymer could be divided into two groups
246
according to the mobility of chain segment, which were RA and MA. The proportion of MA
247
could be estimated by mathematical calculation, and the results were summarized in Table 1. The
248
increased proportion of MA corresponded to the elevated content of movable chain segments in
249
PLA-C-155, which meant that more plastic deformation might occur during tensile process.
250
3.3. The WAXD graphs of PLA films
251
In order to furthermore characterize the structure evolution of SPC PLA film during blow
252
molding, PLA-A-180 and PLA-C-155(110) were chosen to measure WAXD. Performing peak
253
fitting using PeakFit sofeware, the result indicated that PLA-A-180 was amorphous, just as Fig.
254
4a shown. For PLA-C-155(110) (Fig. 4b), mesophase and crystals appeared besides amorphous
255
phase, and the contents of them could be calculated by the area ratio of the diffraction peaks
256
from relevant phase and whole diffraction peaks. Combined DSC data, we believed that
257
mesophase and cohesive entanglement composed RAF, corresponding results were summarized
258
in Table 2. When extrusion temperature was set at 155 °C, extrudate of crystalline PLA still had
13
259
Fig. 4. Measured and fitting WAXD intensity profiles of PLA films:
260
(a)PLA-A-180; (b) PLA-C-155(110)
261
Table 2 Micro-structure contents of films
262 263 264 265
XMA
XRA
XC-DSC
Xmeso
XC-WAXD Xcohesive
(%)
(%)
(%)
(%)
(%)
(%)
PLA-A-180
57.94 40.76
1.30
0.00
-
40.76
PLA-C-155(90)
68.58 25.30
6.12
23.81
6.83
1.49
PLA-C-155(110) 71.88 23.98 PLA-C-155(120) 67.65 27.86
4.14 4.49
21.24 21.61
5.25 4.60
2.74 6.25
266
residual crystals (as shown in DSC data). These micro-crystals induced meosphase and oriented
267
crystallization during flow molding, which was proved by increasing mesophase and crystallinity.
268
It was mesophase that promoted the crystallization capacity of PLA-C-155, which could be
269
verified by decreased Tcc in DSC experiments. At the same time, cohesive entanglement was a
270
main reason of brittle fracture in PLA-A-180, which was effectively suppressed by mesophase
271
(decrease from 40.76 to 2.74 % for PLA-C-155(110)). (Chen, Y et al, 2018) In a word, residual crystals,
272
as a regulator, structured ductile PLA film by effectively adjusting micro-structures.
273
3.4. Mechanical properties of PLA films
274
Next, we characterized the mechanical properties of films and explored the relationship
275
between structure and properties. The amount of crystals decreased while the size increased as
14
276
increasing annealing temperature. Table 3 revealed that decreasing annealing temperature was
277
more favorable for increasing tension toughness of films, which displayed as bigger elongation at
278
break (ε). That attributed the smaller size of crystals to more effective depression of cohesional
279
entanglements during film blowing (The Xcohesive of PLA-C-155(90) was the smallest). The
280
stress-strain curves of PLA films were displayed in Fig. 5. As we could see, PLA-A-180 was
281
hard and brittle film, having high modulus (E) and low elongation at break. It was noticed that
282
PLA-C-155(90, 110 or 120) showed well-defined yield points, strain softening and strain
283
hardening behavior until fracture. Neck-in and plastic flow beyond the yield point could be
284
observed during overall tension process. The E was defined as a ratio of stress to strain in the
285
elastic limit, which was associated with the RA and crystalline phase. PLA-A-180 had higher
286
tensile modulus than other SPC PLA films because it had a higher proportion of cohesive
287
Fig. 5. Stress-strain curves of PLA films along different drawing directions.
288
Table 3. The mechanical properties of PLA films
Materials PLA-A-180 PLA-C-155(90)
E (GPa)
σTS (MPa)
ε (%)
TD/MD
TD/MD
TD/MD
3.41±0.11/4.42+0.34 36.67±4.32/57.52±3.51
2.63±0.83/3.52±1.64
2.42±0.06/4.09±0.24 44.54±3.65/50.22±4.37 113.87±5.73/126.76±5.38
PLA-C-155(110) 2.33±0.13/3.90±0.02 45.42±2.04/78.01±3.43
67.50±3.42/104.83±4.85
PLA-C-155(120) 2.53±0.22/4.11±0.25 33.92±1.73/41.05±2.96
36.42±2.63/99.08±3.46
15
289
entanglement (40.76%). Besides, SPC PLA films had better ability of plastic deformation, which
290
was proved by lower Tg, Xcohesive and higher XMA. And thus, the phenomenon of necking and
291
strain hardening were clearly observed in SPC PLA film during stretching. The increased tensile
292
strength (σTS) of SPC PLA films attributed to the tensile crystallization and strain hardening.
293
Specifically, for PLA-C-155(110), the elongation at break increased to 67.50 % and 104.83 %
294
along TD and MD, respectively. Meanwhile, the tensile strength increased from 36.67 MPa and
295
57.52 MPa to 45.42 MPa and 78.01 MPa along TD and MD, respectively. The detailed data were
296
summarized in table 3. It was the particular morphology structure caused by special processing
297
technology improved the mechanical properties of SPC PLA film.
298
3.5. Thermal shrinkage behaviors of PLA film
299
The improved tensile toughness of films attributed to the increased movability of chain
300
segments. And it was obvious to observe the anisotropy of tensile performance (as seen in Fig. 5),
301
which was usually ascribed to orientation of amorphous or crystalline phase. The PLA-C-155
302
(110) was chosen to investigate the influence of blow molding on orientation. The shrinkage (η)
303
was used to associate the tensile properties and oriented structure of film. Thermal stimulated
304
shrinkage, which was due to the increase of internal energy or stress in molecular level, mainly
305
occurred in oriented system. The orientation of chain segments was a manifestation of frozen
306
internal stress, which might relax and disorient once heating at proper temperature. Generally,
307
chain segments began to move when the temperature was higher than Tg. So, the orientation
308
Table 4. Shrinkage and tension properties of films
309 310
η along TD(%) η along MD(%) PLA-A-180
15.05
13.11
PLA-C-155(110)
21.00
32.34
311
16
Fig. 6. SEM micrographes of etched films: (a) PLA-C-155(110); (b) PLA-A-180; (3) PLA-A-155; (4) PLA-C-180(110) 312
degree of chain segments in non-crystalline region could be evaluated by shrinkage ratio at 70 oC.
313
Table 4 displayed that the shrinkage of PLA-C-155(110) was bigger than PLA-A-180, which
314
meant more oriented chain segments relaxed. Characterizing film shrinkage provided valuable
315
information exploring the evolution of condensed state during film-blowing. For PLA-C-
316
155(110), the increase of shrinkage ratio reflected the reinforced orientation of chain segments in
317
amorphous region and increase of frozen internal stress caused by residual crystals and lowered
318
melting temperature. Particularly, when film blowing was executed at a temperature near melting
319
point, lower movement ability of chains make PLA easily orient. When polymer molecules
320
exited the annular die, the pulling and inflating force tend to orient them along MD and TD,
321
respectively. And the orientation of chain segments along MD was bigger than TD as a result of
322
bigger pulling force. After thermal treatment of 70 oC, chain segments relaxed, and therefore P
323
LA-C-155(110) shrank.
324
3.6. The SEM images of film
325
The movability of MA was stronger than RA and crystals, which was more easily etched by a
326
water-methanol mixture solution. And SEM was used to vividly observe the orientation of film
327
(Fig. 6). The SEM micrograph of PLA-C-155(110) was chosen to display morphologies of SPC
17
Fig. 7. Schematic representation of constructing SCP PLA film using residual crystals and processing temperature of 155 °C 328
PLA film. As seen in Fig. 6a, PLA-C-155 displayed orientation structure in micro-scale.
329
However, PLA-A-180 showed nearly isotropy after etching (Fig. 6b). Combined with the film
330
shrinkage, the chain segments of PLA-A-180 oriented along TD and MD, which was discordant
331
with SEM picture. That was because that the orientation of PLA-A-180 was smaller and unstable,
332
which was destructed during etching process. In contrary, the orientation of PLA-C-155 was
333
effectively fixed by oriented crystals and tensile crystallization. So, the thread-like layers with
334
length as long as several micrometers were perfectly arranged along MD, interpenetrating some
335
amount of layers along TD. Neither PLA-A-155 nor PLA-C-180 displayed oriented
336
morphologies (Fig. 6c and d), indicating that crystalline region and lower processing temperature
337
(near melting point) were necessary to form the special structure.
338
3.7. Mechanism of toughness
339
Due to highly entangled nature of polymer chains, crystals or other factors, the movability of
340
some amorphous chain segments were restricted, namely RA. Based on the relationship between
341
structure and properties, we believed that polymer contains crystalline phase, MA and RA.
18
342
According to the concept of the RA, it mainly divided into mesophase and cohesive
343
entanglement. It was the cohesive entanglement network of polymer chains that leads to the
344
brittle of PLA film, which could be restrained by mesophase and orientation structure.[43] Fig. 7
345
illustrated the process of structuring mesophase and orientation structure. The crystal
346
morphologies of PLA granules were various at different isothermal crystallization temperature
347
range. Generally, lower crystallization temperature corresponded to the more quantity of crystals
348
with smaller size as well as more defects. The molten PLA still included some residual
349
microcrystals as the result of processing temperature lower the completely melting temperature.
350
When the melt exited the annular die, these residual crystals served as regulators to induce the
351
formation of mesophase and tensile crystallization under the effects of stretching due to the
352
pulling rollers and compressed air. The mesophase depressed the cohesive entanglement, which
353
was proved by WAXD and DSC. So, increased MA and mesophase endowed the PLA chains
354
with excellent plastic flow. Beyond that, the mobility of chains was lower at 155 °C than 180 °C,
355
which contributed to induced orientation structure. Meanwhile, the residual microcrystals and
356
tensile crystallization fixed the partial orientation, which were well coincided with the results of
357
shrinkage and SEM. The oriented but un-crystallized MA better transferred stress and have the
358
tendency to relax. According to Men and Wang, the tensile toughness of PLA film improved by
359
constructing a firmly amorphous network with increased chain movability.[44,45] Lower
360
crystallization temperature, the more residual microcrystals the polymer melt has, which more
361
effectively regulated condensed state and so improved the mechanical properties of film.
362
4. Conclusion
363
Although all polymers comprised of crystalline and non-crystalline phase, not all polymer
364
could be termed as SPC. It was due to that the crystals could not combine with orientation. After
19
365
isothermal crystallization and film molding near melting point, we preserved residual
366
microcrystals and then they, as regulators, effectively induced mesophase and tensile
367
crystallization. The mesophase depressed cohesive entanglements, which improved plastic
368
deformation of chains. Meanwhile, crystals fixed the orientation caused in the film inflating
369
process, and the oriented network effective transferred stress during stretching. The tensile
370
toughness of PLA film was greatly increased by structuring the special condensed state. It is an
371
excellent example to understand the relationship of structure and properties.
372
Appendix A. Supplementary data
373
The following is Supplementary data to this article
374
Author information
375
Corresponding Authors
376
* E-mail: [email protected],
377
** E-mail: [email protected].
378
Notes
379
There are no conflicts of interest to declare.
380
Acknowledgment
381
This work was financially supported by the Science and Technology Bureau of Jilin Province of
382
China (No. 20170204043GX) and the National Natural Science Foundation of China (No.
383
51503204)
384
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Highlights: Residual microcrystals induced mesophase and tensile crystallization Mesophase depressed cohesive entanglements and improved plastic deformation of chains. The crystal-cross linked network effective transferred stress
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
There are no conflicts of interest to declare.