Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES
Article
Polylactide/poly(#-hydroxytetradecanoic acid) reactive blending: a green renewable approach to improving polylactide properties Stephen Spinella, jiali cai, Cedric Samuel, Jianhui Zhu, Scott A McCallum, Youssef Habibi, Jean-Marie Raquez, Philippe Dubois, and Richard A Gross Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.5b00394 • Publication Date (Web): 07 Apr 2015 Downloaded from http://pubs.acs.org on April 14, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Biomacromolecules is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Biomacromolecules
Polylactide/poly(ω-hydroxytetradecanoic acid) reactive blending: a green renewable approach to improving polylactide properties
1 2 3 4
Stephen Spinella,a,b,c Jiali Cai, b Cedric Samuel,c Jianhui Zhu,a,b Scott A. McCallum,a Youssef
5
Habibi,c Jean-Marie Raquez,c Philippe Dubois,c Richard A. Gross* a
6
a
7
Chemical Biology, Rensselaer Polytechnic Institfute, 110 8th Street, Troy, N.Y. 12180, USA
8
Center for Biotechnology and Interdisciplinary Studies and Department of Chemistry and
b
9 10
Department of Chemical and Biomolecular Engineering, NYU Polytechnic School of Engineering, 6 Metrotech Center, Brooklyn, New York 11201, USA.
c
Centre d’Innovation et de Recherche en MAtériaux Polymères CIRMAP, Service des
11
Matériaux Polymères et Composites, University of Mons, Place du Parc 23, B-7000 Mons,
12
Belgium
13 14 15 16
*Corresponding Author
17
Richard A. Gross; Email:
[email protected]; Ph: 518-276-3734; Fax: 518-276-3405
18 19
KEYWORDS: reactive blending, polylactide, poly(ω-hydroxytetradecanoic acid), block
20
copolymer, titanium tetrabutoxide
ACS Paragon Plus Environment
1
Biomacromolecules
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 30
1
ABSTRACT
2
A green manufacturing technique, reactive extrusion (REx), was employed to improve the
3
mechanical properties of polylactide (PLA). To achieve this goal, a fully bio-sourced PLA based
4
polymer blend was conceived by incorporating small quantities of poly(ω-hydroxytetradecanoic
5
acid) (PC14). PLA/PC14 blends were compatibilized by transesterification reactions promoted
6
by 200 ppm titanium tetrabutoxide (Ti(OBu)4) during REx. REx for 15 min at 150 RPM and 200
7
°C resulted in enhanced blend mechanical properties while minimizing loses in PLA molecular
8
weight. SEM analysis of the resulting compatibilized phase-separated blends showed good
9
adhesion between dispersed PC14 phases within the continuous PLA phase. Direct evidence for
10
in situ synthesis of PLA-b-PC14 copolymers was obtained by HMBC and HSQC NMR
11
experiments. The size of the dispersed phase was tuned by the screw speed to ‘tailor’ the blend
12
morphology. In the presence of 200 ppm Ti(OBu)4, inclusion of only 5% PC14 increased the
13
elongation at break of PLA from 3 to 140 % with only a slight decrease in the tensile modulus
14
(3200 to 2900 MPa). Furthermore, PLA’s impact strength was increased by 2.4 times that of neat
15
PLA for 20 % PC14 blends prepared by REx. Blends of PLA and PC14 are expected to expand
16
the potential uses of PLA-based materials.
17 18 19 20 21
ACS Paragon Plus Environment
2
Page 3 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
Biomacromolecules
INTRODUCTION
2
Volatility of petroleum prices and concerns over climate change has led to increased interest in
3
the development of sustainable polymers from readily-renewable resources. Polylactide (PLA) is
4
currently one of the most extensively researched and utilized biodegradable and renewable
5
thermoplastic polyesters. Further improvements in its material properties will enable deeper
6
market penetration and, consequently, replacement of larger quantities of conventional
7
petrochemical-based polymers in many applications (packaging, electronics, automotive, etc.).
8
Despite numerous advantages, limitations in PLA properties include low toughness, poor gas-
9
and water-impermeability, slow crystallization rate, and low thermal stability.1 The low ductility
10
of PLA (elongation at break of 3 %) impedes its use in flexible packaging.1–3
11
Plasticization has been used to increase PLA’s processability, ductility and toughness.4,5
12
Common small-molecule PLA plasticizers include citrate ester derivatives that both decrease
13
PLAs glass transition temperature (Tg) and increase its ductility.6,7 Low molecular weight
14
polyesters have also proved to be effective additives for increasing PLA ductility.8,9 To reduce
15
problems of plasticizer migration, phase separation and leaching,10 plasticizer such as citrate
16
esters were grafted onto PLA by reactive extrusion with malic anhydride-graft-PLA.11 However,
17
plasticizing PLA limits it’s applications due to the aforementioned plasticizer leaching and
18
decreased Tg values which restricts its use at higher temperatures.
19
Copolymerization represents another approach to develop PLA-based materials with improved
20
tensile and impact properties. Indeed, copolymerization of L-lactide (L-LA) has been extensively
21
explored. For example, copolymers of ε-caprolactone (CL),12,13 trimethylene carbonate,14 δ-
22
decalactone15 were successfully synthesized and used to tune material mechanical properties.
23
However, copolymerization generally reduced the materials Tg and tensile strength. Furthermore,
ACS Paragon Plus Environment
3
Biomacromolecules
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 30
1
the development of multiple copolymer products is not preferred in industrial settings where the
2
desire is to manufacture larger volumes of PLA materials that can be modified via extrusion
3
processes giving final products.4
4
Polymer blending represents an industrially relevant strategy to increase PLA’s elongation at
5
break. Previous literature reports describe blending of PLA with poly(ε-caprolactone) PCL,16–18
6
poly(butylene succinate) PBS,19,20 poly(butylene succinate-co-adipate) PBSA,21 poly(butylene
7
adipate-co–terephthalate) PBAT22 and polyhydroxyalkanoates (PHAs).23,24 However, as most
8
polymer blends are thermodynamically immiscible,25 phase separation occurs leading to a
9
decrease in material properties due to poor interfacial adhesion. Recently, plant based low Tg
10
polyesters produced from hydrogenated dimers of C18 fatty acids, sebacic acid and 1,10-
11
decanediol were directly melt blended with PLLA and large increases in PLA strain at break
12
were observed.26,27 For 10% contents of terpolymers with variable fatty acid dimer contents,
13
strain at break values from 63 to 385% were attained. However, the modulus and stress at yield
14
decreased by about 50 and 20%, respectively. Furthermore, increases in the notch IZOD impact
15
strength is by about 4.2 fold relative to neat PLA was reported at 20% blend contents. Blend
16
morphology and properties was found to be highly dependent on the terpolyester composition.
17
In an effort to increase adhesion between the two phases, blending has been performed in the
18
presence of interfacial compatibilizers such as P[LA-b-CL] diblock copolymers in the case of
19
PLA/PCL blends.28,29 However, since the manufacture of diblock interfacial compatibilizers is
20
often expensive, this remedy to improve blend compatibility is often not adopted by industry.
21
Recent work has demonstrated that PLA/PBS blends can be compatibilized by incorporating
22
corresponding copolymers. Shibata et al.20 showed that poly(BS-co-LA) with low contents of
23
lactate units effectively compatibilized PLA/PBS blends giving materials that followed the rule
ACS Paragon Plus Environment
4
Page 5 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Biomacromolecules
1
of mixtures except that their elongation at break values were exceptionally higher than pure
2
PLA. To obtain fully bio-based and biodegradable blends, PLA was also blended with microbial
3
polyesters or polyhydroxyalkanoates (PHAs). The mechanical properties of PHAs can be tuned
4
by varying the content and composition of (co)monomers incorporated along the backbone.30
5
The majority of PLA/PHA blends prepared by solvent casting or melt-blending were
6
immiscible.31,32 However, adding 20 wt% of a random 3-hydroxybutyrate/3-hydroxyhexanoate
7
copolymer to PLA resulted in a toughening effect.33
8
Our research team developed a mild and efficient biotechnological route to convert fatty acids,
9
such as tetradecanoic acid, to ω-hydroxyfatty acids that are building blocks to synthesize
10
polyesters.34,35 An engineered Candida tropicalis strain was developed whose background
11
activity for conversion of ω-hydroxyfatty acids to its corresponding ω-carboxydiacids was
12
dramatically reduced. To achieve this goal, 16 genes were identified and removed from the C.
13
tropicalis genome including 6 cytochrome P450s, 4 fatty alcohol oxidases, and 6 alcohol
14
dehydrogenases. After reinsertion of a suitable cytochrome P450 (CYP52A17) enzyme, the new
15
C. tropicalis strain produced over 160 g/L ω-hydroxytetradecanoic acid from the C14-methyl
16
ester with less than 5% formation of the corresponding diacid.34 Subsequently, ω-
17
hydroxytetradecanoic acid was converted by a condensation polymerization catalyzed by
18
titanium tetraisopropoxide (Ti[OiPr]4) to poly(ω-hydroxytetradecanoic acid) (PC14) with Mw up
19
to 140K.35 Tensile tests of PC14 with Mw above 78K showed a strain-hardening phenomena and
20
tough properties with elongation at break of about 700% and true tensile strength up to 63 MPa.
21
Furthermore, DSC analysis revealed PC14 has a melting temperature at about 94 °C and is
22
highly crystalline.34 Recent work has also shown that PC14 films are readily compostable by
ACS Paragon Plus Environment
5
Biomacromolecules
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 30
1
ISO14855 (unpublished). In addition, these materials rapidly crystallize from the melt36,37 and
2
have excellent hydrolytic stability.38
3
Unfortunately, like most polymer blends, blends of PC14 and PLA are thermodynamically
4
immiscible due to their relatively low amount of ester linkages along the PC14 backbone.
5
Consequently, phase separation of PLA and PC14 occurs. Furthermore, weak adhesion between
6
these immiscible phases and poor stress transfer results in brittle materials. Reactive extrusion
7
has been investigated as a green solvent-less method to compatibilize polymer blends. For
8
example, Wang et al.39 reported that the elongation at break of PLA/PCL blends was increased
9
from 28% to 127% by adding 2% of a chain extending agent, triphenyl phosphite, (TPP) during
10
blending. Similarly, Ojijo et al.40 reported the compatibilization of PLA/PBSA blends during
11
blending by addition of TPP. Optimization of reaction parameters for blends containing 10 and
12
30 wt% PBSA with 2 wt% TPP resulted in materials where the impact strength was increased
13
from 6 kJ/m2 for PLA to 11 and 16 kJ/m2 and the elongation at break increased from 5% for PLA
14
to 20 and 37%. Coltelli et al. reported that Ti(OBu)4 is a useful catalyst for compatibilization of
15
PLA/PBAT during melt blending. Reactive blending of PLA/PBAT (75/25) with 0.7 wt%
16
Ti(OBu)4 for 40 min resulted in increases in the strain at break from 10 to 80 % relative to pure
17
PLA. Furthermore, changes in blend morphology consistent with compatibilization were
18
observed. Also, Liu et al.41 reported that Ti(OBu)4 catalyzed transesterification reactions during
19
melt blending of PLA with polycarbonate (PC) by the in situ formation of P[LA-co-PC]
20
copolymers.
21
This paper describes the use of reactive extrusion (REx) for blending poly(ω-hydroxyfatty
22
acid) bioplastic (PC14) with PLA to improve PLA’s ductility and impact strength without
23
effecting other valued PLA properties. While PCL, PBSA and PBAT have been studied for this
ACS Paragon Plus Environment
6
Page 7 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Biomacromolecules
1
purpose with some success, substantial room for improvement exists. Most importantly, PLA
2
blends with PCL, PBSA and PBAT fall short of the goal of using low amounts of a polymer in
3
blends with PLA to achieve large improvements in ductility with little change in Young’s
4
modulus and stress at yield. Also, while there has been significant progress in developing cost-
5
effective routes to biobased butane diol and succinic acid,42 polymers containing these building
6
blocks are primarily manufactured from petrochemicals as is PCL, PBS and PBSA. While at an
7
early stage, efforts are underway to commercialize the production of ω-hydroxyfatty acids by
8
SyntheZyme and the corresponding polyesters have valuable mechanical properties35–37,42,43. The
9
use of reactive extrusion (REx) for compatibilizing blends of PLA and PC14 is a promising
10
‘green’ processing method that is solvent-free and can be run in a continuous mode. This paper
11
describes studies on the compatibilization of PC14 and PLA by melt-blending using a mini-DSM
12
microcompounder and Ti(OBu)4 as the transesterification catalyst.44
13
resulting blends were studied using SEC, tensile testing and microscopy.
The properties of the
14 15
Materials and Methods
16
Materials. A commercially available extrusion-grade PLA (NatureWorks 2002D) was used as
17
received (Mn = 134,000 ± 5000 g/mol, Đ (dispersity)
18
exclusion chromatography, 4.4 ± 0.2% D-isomer content according to the supplier). Poly(ω-
19
hydroxytetradecanoic acid), PC14, was kindly provided as a gift from SyntheZyme LLC and was
20
used as received (Mn = 75,000 ± 5000 g/mol, Đ = 3.1 ± 0.1 as determined by size-exclusion
21
chromatography). Titanium tetrabutoxide, Ti(OBu)4, reagent grade (97%, Sigma), was used as
22
received.
23
Sample Preparation and Reactive Extrusion
=
1.94 ± 0.06 as determined by size-
ACS Paragon Plus Environment
7
Biomacromolecules
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 30
1
PC14 and a quantity of Ti(OBu)4 such that the corresponding blend contains 200 ppm catalyst
2
were mixed in dry CHCl3 and films were prepared by solvent casting. The solvent was removed
3
(3 h ambient conditions, 12 h at 80 °C at 10-1 mbar) and films were cut into squares (about 1 mm
4
by 1 mm). Then, PC14 premixed with Ti(OBu)4 and an amount of PLA corresponding to the
5
pre-determined blend composition were introduced into a DSM twin-screw micro-compounder
6
(15 cc, Xlpore, Holland) and mixed at 30 RPM in 3 min. Subsequently, the screw speed was
7
increased to a predetermined mixing speed for a pre-determined time before being extruded out
8
of the dye. As a control, a binary blend exclusively made of PLA and PC14 without the addition
9
of catalyst was prepared using a DSM twin-screw micro-compounder (15 cc, Xlpore, Holland)
10
under similar operating conditions. For tensile and impact characterizations, the PLA-based
11
materials were prepared by injection molding with the oven temperature set at 200 °C and the
12
mold temperature set at 50 °C for 4 min.
13
Gel Permeation Chromatography
14
Size-exclusion chromatography (SEC) was performed in chloroform (sample concentration: 1
15
wt.%) at 35°C using a polymer laboratories (PL) liquid chromatography equipped with a PL-
16
DG802 degasser, an isocratic HPLC pump LC1120 (flow rate: 1 mL/min), a Basic-Marathon
17
Autosampler, a PL-RI refractive index detector and three columns: a PLgel 10 lm guard column
18
(50 x 7.5 mm) and two PLgel mixed-B 10 lm (300 x 7.5 mm) columns. Molar mass and molar
19
mass distribution values were calculated by reference to a relative calibration curve made of
20
polystyrene standards.
21
Differential Scanning Calorimetry
22
Differential scanning calorimetry (DSC) was performed using a TA Instruments 2920 CE
23
Modulated DSC. The instrument was equipped with a refrigerated cooling system (RCS) for
ACS Paragon Plus Environment
8
Page 9 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Biomacromolecules
1
quasi-isothermal experiments, whereas, a liquid nitrogen cooling system (LNCS) was used for
2
non-isothermal experiments. Temperature and enthalpy calibration were performed using an
3
indium standard; heat capacity calibration was performed using sapphire disks. The glass
4
transition temperature (Tg), crystallization temperature (Tc), enthalpy of crystallization or cold
5
crystallization (∆Hc) and melting enthalpy (∆Hm) were determined from the second heating and
6
cooling scans performed at 10 °C/min under nitrogen flow.
7
Tensile Testing
8 9 10 11
Tensile tests were performed on injection molded samples according to ASTM D638 using a Zwick universal tensile testing machine (speed =3 m/s and preload = 5 N). Izod Impact Testing Notched Izod impact tests were performed according to ASTM D256 using a Ray-Ran 2500
12
pendulum impact tester (E = 4 J, mass = 0.668 kg and speed = 0.46 m/s).
13
Scanning Electron Microscopy
14
The extrudate was fractured in liquid nitrogen, the fractured ends were sputter-coated with
15
gold and then examined for morphological structure by scanning electron microscopy (SEM)
16
carried out using a Philips XL20 microscope (20 kV).
17
NMR Spectroscopy
18
All NMR spectra were obtained at 25 °C using Bruker 600 and 800-MHz spectrometers
19
equipped with a 1H⁄13C⁄15N cryoprobe and z-axis gradients. Data were acquired, processed, and
20
analyzed using Bruker TopSpin 2.1 software. Samples typically contained approximately 10
21
mg/mL of blend material dissolved in CHCl3-d.
22
heteronuclear single quantum coherence (HSQC) and heteronuclear multiple bond correlation
1D proton, 2D 1H-1H NOE, 2D 1H-13C
ACS Paragon Plus Environment
9
Biomacromolecules
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 30
1
(HMBC) spectra were acquired for a series of samples including those containing the catalyst
2
before and after having undergone extrusion.
3
RESULTS AND DISCUSSION
4
Blends of PLA and PC14 were compatibilized by an industrially relevant and environmentally
5
friendly process, reactive melt blending, to increase the potential applications for PLA based
6
materials. To achieve this goal, the variable set studied for reaction extrusion experiments was as
7
follows: screw speed, residence time within the extruder, reaction temperature and PLA/PC14
8
blend ratio. The screw speed was varied to investigate the effect of mixing rate on the dispersed
9
phase while trying to minimize depolymerization. Increased mixing will decrease the dispersed
10
phase size while increasing the surface area between the two phases for transesterification
11
reactions.45 The residence time in the extruder was varied to determine the minimum time
12
needed to attain a compatibilized blend. Different reaction temperatures were studied to identify
13
a
14
depolymerization reactions. Changing the blend composition provided a means to tune the blend
15
properties from brittle to ductile. First, variation of the screw speed was explored while the
16
reaction temperature, residence time, catalyst concentration and blend ratio were fixed at 200 oC,
17
30 min, 200 ppm Ti(OBu)4 and PLA/PC14 (90/10 w/w), respectively. A 30 min reaction time
18
was selected for the first set of experiments from results based on a set of preliminary PLA/PC14
19
melt-blending experiments performed in round bottom flasks with overhead mixing (data not
20
reported herein). The selection of screw speeds ≥ 60 RPM was chosen based on earlier work by
21
our research team on PLA melt-blending.46 The influence of screw speed on product molecular
22
weight and tensile properties is shown graphically in Figure 1 and the corresponding values are
23
listed in Table SI-1. First, comparison of the two experiments performed at 60 RPM’s shows that
temperature
that
optimizes
catalyst
transesterification
activity
while
minimizing
ACS Paragon Plus Environment
10
Page 11 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Biomacromolecules
1 2
Figure 1. Effect of screw speed on blend molecular weight and elongation at break for (PLA
3
90/10 w/w) prepared by reactive extrusion with a residence time, screw speed and catalyst
4
concentration at 30 min, at 200 oC of 200 ppm of Ti(OBu)4
5
the presence of Ti(OBu)4 in extrusions results in extruded product of lower Mw. Further study of
6
Figure 1, where the concentration of Ti(OBu)4 remains constant and the screw speed is
7
increased, indicates that both these factors are responsible for decreased extruded product
8
molecular weight. Indeed, it is well known that higher screw speeds results in a combination of
9
both mechanical degradation and heat evolution caused by the torque generated from the screws
10
that can result in polyester chain cleavage events.47 Consistent with above, a regular trend was
11
observed where higher screw speeds result in products of lower Mw values. A control blend
12
without catalyst was conducted at 150 RPM and, indeed, a 10% decrease in Mw was observed
13
and attributed to thermo-mechanical effects. Values of Đ remained at about 2.1 but increased at
ACS Paragon Plus Environment
11
Biomacromolecules
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 12 of 30
1
≥180 RPM to ≥ 2.7. For example, when the screw speed was increased from 150 to 180 RPM,
2
the result was a 27% decrease in Mw (to 165 kg/mol) and an increase in Đ from 2.1 to 2.7.
3
Increase in the screw speed from 180 to 200 RPM resulted in no significant change in Mw and Đ.
4
However, further increase in the screw speed to 220 RPM caused a further decrease in Mw to 150
5
kg/mol. Mechanical property changes as a function of the screw speed were also investigated and
6
corresponding values of tensile modulus and elongation at break are listed in Table SI-1.
7
Increasing the screw speed from 60 to 150 RPM led to higher values of elongation at break. This
8
is attributed to that, increasing the screw speed decreases the size of the reinforcing phase (PC14)
9
which increases the probability of a transesterification reaction.48 A control experiment at 150
10
RPM without catalyst, further confirmed the effect of the catalyst in improving blend ductility
11
since, without catalyst, no significant increase in elongation at break was observed (elongation at
12
break = 20 %, see Table SI-1). A screw speed of 150 RPM was optimal in terms of blend
13
ductility, giving an elongation at break of 152 %. Increasing the screw speed above 150 RPM led
14
to decreases in average elongation at break values albeit these decreases are within experimental
15
error. The Young’s modulus of the blend remained constant (about 2900 MPa) regardless of the
16
screw speed up until 200 RPM. However, an increase in the screw speed to 220 RPM resulted in
17
a significant decrease in the Tensile Modulus to 2750 MPa. The cumulative results from Figure 1
18
led us to conclude that 150 RPM should not be exceeded under the current reaction conditions
19
since; above that value, deleterious effects on Mw and/or Đ as well as blend mechanical
20
properties were observed.
21
The potential that shorter residence times would suffice was investigated. The residence time
22
was varied from 5 to 30 min while the reaction temperature, mixing speed and catalyst
23
concentration was held constant (200 °C, 150 RPM and with 200 ppm Ti(OBu)4, respectively).
ACS Paragon Plus Environment
12
Page 13 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Biomacromolecules
1
Results listed in Table SI-2 showed that Mw decreased by 11% (280 to 250 kg/mol) with increase
2
in the residence time from 5 to 15 minutes. Further increases in residence time lead to minor
3
changes in Mw, although, increases in Đ from 2.2 to 2.7 were observed. Mechanical properties
4
were studied as a function of reaction time to ascertain the minimum time needed to generate
5
sufficient adhesion between the PLA and PC14 phases in order to create a ductile blend.
6
Increasing the reaction time from 5 to 10 min increased the elongation at break from 50 ± 30 to
7
90 ± 30, respectively, as shown in Table SI-2. Further increase in the reaction time to 15 min
8
resulted in an elongation at break of 145 ± 35. Additional increase in time beyond 15 min did not
9
lead to a corresponding increase in the blend elongation at break. Hence, 15 min was selected as
10
the preferred residence time for further studies below.
11 12
Figure 2. Stress/strain curves for PLA/PC14 blends prepared by reactive extrusion at
13
temperatures from 180 to 220 oC while fixing the residence time, screw speed and catalyst
14
concentration at 15 min, 150 RPM and 200 ppm Ti(OBu)4, respectively.
15
Further investigation of reactive processing conditions was performed by varying the
16
reaction temperature from 180 to 220 oC while fixing the residence time, mixing speed and
17
catalyst concentration at 15 min, 150 RPM and 200 ppm Ti(OBu)4, respectively (see Figures 2, 3
ACS Paragon Plus Environment
13
Biomacromolecules
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 14 of 30
1
and Table SI-3). Increasing the temperature from 180 to 200 oC resulted in a 20% decrease in
2
Mw while Đ remained unchanged. However, over this temperature interval, the %-elongation-at-
3
break increased from 40 to 160% indicating an increase in the occurrence of transesterification
4
reactions required to compatibilize immiscible PLA and PC14 phases. In contrast to the 200 oC
5
run performed with catalyst, the extruded product without catalyst was brittle (elongation-at-
6
break 3%). This provides important evidence that the catalyst is responsible for promoting
7
transesterification reactions resulting in the in situ generation of block copolymer compatibilizer
8
molecules. Further increasing the reaction temperature to 210 and 220 oC with catalyst addition
9
gave further improvements in the elongation at break although Mw decreased by 27% at 220 oC
10 11
Figure 3. Effect of temperature on the molecular weight and elongation at break of PLA/PC14
12
(90/10 w/w) blends prepared by reactive extrusion with a residence time, screw speed and
13
catalyst concentration at 15 min, 150 RPM and 200 ppm Ti(OBu)4, respectively.
ACS Paragon Plus Environment
14
Page 15 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Biomacromolecules
1
relative to the reactive extrusion process at 180 oC. The tensile modulus was not significantly
2
altered (~2900 MPa) for reaction temperatures between 180 and 220 oC. Consequently, 200 oC
3
was selected as the preferred temperature.
4 5
Figure 4. SEM images of cryofractured PLA/PC14 (90/10) prepared by melt blending (at 200°C,
6
screw speed 150 RPM, residence time 15 min) without (a) and with (b) 200 ppm of Ti(OBu)4
7
The morphology of blends, with and without Ti(OBu)4, was examined by SEM on
8
cryofractured samples (Figure 4). Without Ti(OBu)4, little adhesion between the matrix (PLA)
9
and the reinforcing phase (PC14) was observed, as many voids are seen in the SEM due to pull-
10
out or detachment of weakly held PC14 dispersed phases from the PLA matrix (Figure 4a). In
11
contrast, SEM images of compatibilized blends prepared using 200 ppm of Ti(OBu)4 as catalyst
12
(Figure 4b) show that the connection(s) between the continuous and dispersed phases are
13
strengthened since the two phases remain adhered with little formation of empty pits.
14
Consequently, efficient stress transfer between the matrix (PLA) to the rubbery phase (PC14) is
15
achieved that results in increased blend ductility reflected in elongation at break values. The
16
observed difference in morphology between the uncompatibilized and compatibilized blends is
17
attributed to formation by Ti(OBu)4 catalyzed transesterification reactions of PLA-b-PC14.
ACS Paragon Plus Environment
15
Biomacromolecules
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 16 of 30
1
These block copolymers, which must reside to some extent at the interface of phase domains,
2
promote adhesion between PC14 and PLA phases.
3 4
Figure 5: HSQC (A) of PLA/PC14 (80:20) blend prepared without (black) and with (red)
5
Ti(OBu)4 ; (B) HMBC of blend prepared without Ti(OBu)4 (peaks H18C16 and H14C1) and with
6
Ti(OBu)4 (peaks H18’C1C’ and H14’C14’ (HSQC) and H14’C16’ (HMBC))
7
To obtain direct evidence that in-situ synthesized PLA-b-PC14 interfacial compatibilizers
8
were formed herein, two-dimensional (2D) Nuclear Magnetic Resonance (NMR) experiments
9
(1H-13C HSQC and HMBC) were performed. Both 1H-13C HSQC and HMBC have proven to be
10
useful in elucidating the microstructure of block polymers.49,50 In HSQC and HMBC spectra,
ACS Paragon Plus Environment
16
Page 17 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Biomacromolecules
1
most sites throughout the interior of the polymers were detected as a single set of positional
2
averaged resonances and are assigned according to literature values.51–53 A few minor peaks were
3
detected in both unreacted and reacted samples and assigned to the PLA hydroxyl end-group unit
4
including 1H peaks at 4.35 and 1.4 ppm, and 13C peaks at 66.75 and 16.82 ppm for the methine
5
and methyl groups of PLA, respectively. From HMBC, signals due to the carboxyl end unit of
6
PLA are also observed. Specifically, in Figure 5B, a correlation between this terminal methine
7
group and a shifted carboxylic acid carbonyl carbon is observed (H18”C16”). The assignments
8
of the carboxyl and hydroxyl terminal units of PLA chains are consistent with those previously
9
reported in the literature.54
10
New Peaks in the compatibilized blend (melt-blended with Ti(OBu)4) consistent with the
11
presence of intra-copolymer dyads (i.e. C14-LA) are shown in selected regions of the HSQC and
12
HMBC (complete spectra are provided in supporting information (SI-1 to SI-6)). Specifically,
13
the correlated 1H and 13C resonances at 4.1 and 65.5 ppm of the HSQC and 4.1 and 170.2 ppm of
14
the HMBC spectra are consistent with the chemical shifts reported for a C14 methylene unit
15
linked directly to an LA unit, -(O=C)-(CH2)12CH2O(C=O)-LA (labeled in H14’C14’ and
16
H14’C16’ Figure 5A and B, respectively). The assignment of the aforementioned linkage (-
17
(O=C)-(CH2)12CH2O(C=O)-LA) is based on previous reports of the analogues ε-caprolactone
18
(CL) units linked directly to a LA unit, (i.e. (O=C)(CH2)4CH2O(C=O)-LA) in Poly (LA-co-CL)
19
copolymers.55,56
20
Based on the relative integrals of the C14 methylene peaks, it is estimated that 0.6% of C14
21
units have a neighboring LA unit with directionality PC14-C14-LA-PLA directionality.
22
Additional unique signals were observed in spectra of the compatiblilized blend including those
23
assigned to LA methine protons and LA carbonyl carbons of LA-C14 and/or C14-LA units that
ACS Paragon Plus Environment
17
Biomacromolecules
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 18 of 30
1
appear at 5.0 ppm and 171.5 ppm (H18’C16’ in figure 5B), respectively. The equivalent methine
2
proton and carbon of LA-LA dyads appear at 5.2 and 170 ppm, respectively. In the case of
3
hetero-dyad linkages, multiple carbonyl correlations in the HMBC are anticipated for the
4
methine proton. These would involve the carbonyl within the monomer and the carbonyl of the
5
i+1 monomeric unit. Based on reported chemical shifts for these dyad structures, the C16’H18’
6
correlation is consistent with C14-LA dyads. These assignments are consistent with those
7
previously reported for CL-LA dyads in random P(LA-co-CL) copolymers.57 No evidence was
8
found for LA-C14 dyads whose peaks may overlap with other signals.58 Based on the above
9
NMR and SEC analysis, reactive blending promotes reactions between PLA (matrix) and PC14
10
(reinforcing phase) that can occur by ester-ester exchange, alcoholysis, and acidolysis reactions.
11
Plausible mechanisms depicting intermolecular ester-ester exchange reactions have previously
12
been reported for PC/PBT59,60 and PLA/PC blends.41
13
Differential scanning calorimetry (DSC) was performed on PLA-PC14 blends compatibilized
14
by reactive melt blending using Ti(OBu)4. DSC thermograms are displayed in Figure 6A (first
15
heating scan) and Figure 6B (second heating scan) and the corresponding values for thermal
16
transitions are listed in Tables SI-4A and SI-4B in the supporting information. The glass
17
transition temperature (Tg) of PLA showed only minor variation (61 to 63 °C) for all blend
18
compositions for both first and second heating scans. These results are consistent with a
19
compatibilized immiscible blend consisting of a rubbery phase (PC14) dispersed in the PLA
20
matrix. Similar findings were observed for PLA blends with PBAT,22 Polybutylene carbonate
21
(PBC),61 PCL,46,62 and polyester elastomers containing itaconic acid units.63
22
crystallized rapidly, the Tg of PC14 is not observed by DSC.35 Furthermore, for all blend
23
compositions, the Tm of both polymers are observed at temperatures that are nearly identical to
Since PC14
ACS Paragon Plus Environment
18
Page 19 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Biomacromolecules
1
the corresponding homopolymers. The fact that the blend composition had little effect on the Tm
2
is also consistent with a compatibilized immiscible blend.
3
4 5
Figure 6. DSC of PLA:PC14 blends prepared by reactive extrusion (200°C, 150 RPM, 15
6
minutes): (a) first heating scans (b) second heating scans. DSC runs were performed under N2
7
flow, with a heating rate of 10 °C/min from 0 to 200 °C.
8
DSC scans in Figures 6a and 6b show that the Tm of PC14 is at about 91 °C (Tm of PC14
9
homopolymer is 94 oC).35 The PC14 melting transition is followed by PLA cold crystallization
ACS Paragon Plus Environment
19
Biomacromolecules
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 20 of 30
1
and, subsequently, the PLA Tm at about 150 °C (Tm of PLA homopolymer with 4.5% D-lactide
2
content is 150 oC).64 The Tm of PC14 varied between 88.4-93.5°C, depending on its ratio in the
3
blend. Despite the fact that PC14 crystallizes rapidly, PC14 did not fully crystallize from the
4
melt after reactive extrusion. PLA:PC14 blends with 5%, 10%, 15%, and 20% PC14 have
5
melting enthalpies (∆HPC14) during the first heating scan, calculated by the rule of mixtures, that
6
are 10%, 40%, 44% and 53% of that for neat PC14. After cooling to 0°C from the melt at a
7
cooling rate of 10°C/min, PC14 phases reached crystallinities that are similar to neat PC14. For
8
comparison, thermal analysis was also performed on neat PLA extruded under similar conditions
9
to PLA:PC14 blends (200°C, 150 RPM, 15 minutes). During the first heating scan, a low
10
intensity melting peak is observed at 149.2 °C. Furthermore, after cooling at 10°C/min from the
11
melt, a melting transition is not observed. These results are consistent with that, PLA with 4.5%
12
D-lactide crystallizes slowly from the melt.22,65,66 Overall, the addition of PC14 (5 – 20%) did
13
not significantly influence PLA’s melting transition, which ranged from 149-152°C, or the
14
crystallinity of the PLA phase, which increased from 4.6% for neat PLA to 8.2 and 8.5% for 5%
15
and 20% PC14 blends, respectively (see Figure 6a and Table SI-4A). Furthermore, based on
16
results from second heating DSC scans, PC14 does not significantly promote PLA crystallization.
17
The PLA phase of melt processed blends is either amorphous or has low crystallinity (
