Relationships between Architectures and Properties of Highly

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Relationships Between Architectures and Properties of Highly Branched Polymers: The Cases of Amorphous Poly(trimethylene carbonate) and Crystalline Poly(#-caprolactone) Yingying Ren, Zhiyong Wei, Xuefei Leng, Tong Wu, Yufei Bian, and Yang Li J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b01867 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 13, 2016

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The Journal of Physical Chemistry

Relationships between Architectures and Properties of Highly Branched Polymers: The Cases of Amorphous Poly(trimethylene carbonate) and Crystalline Poly(ε-caprolactone) Yingying Ren, Zhiyong Wei, Xuefei Leng, Tong Wu, Yufei Bian, and Yang Li∗ State Key Laboratory of Fine Chemicals, Department of Polymer Materials, School of Chemical Eng ineering, Dalian University of Technology, Dalian 116024, China

∗

Corresponding author.

E-mail address: [email protected] (Y. Li). Tel.: +86-411-84981006 1

ACS Paragon Plus Environment


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ABSTRACT: Highly branched polymers (HBPs) are a special class of functional polymeric materials and possess unique properties due to their unique topological structure. A new series of highly branched linear-comb and star-comb amorphous poly(trimethylene carbonate)s (PTMC) and crystalline poly(ε-caprolactone)s (PCL) with well-defined structure and high molecular weight were firstly synthesized using hydroxylated polybutadiene (HPB) as macroinitiators by simple “one-step” and “graft from” strategies. It is expected that the impact of long-chain, highly branched architecture on the properties of amorphous and crystalline polymers, respectively, is different. We explored systematically for the first time the effect and comparison of branched architectures on the physical and chemical properties of highly branched PTMCs and PCLs, including the intrinsic viscosity, glass transition, thermal degradation, creep property, rheological property, and crystallization and melting behaviors. It is found that the intrinsic viscosities in solution for both comb branched PTMCs and PCLs were much lower compared with their linear and star counterparts arise from more compact structure and smaller hydrodynamic volumes. For amorphous PTMC, the creep strain and rate increased remarkably with degree of branching increasing due to the shorter side chains making it difficult for the highly branched molecules to entangle. For crystalline PCL, both WAXD and DSC analysis of PCLs with different topological structures indicated that the comb branched architectures have no significant influence on the crystal structure of PCL, but greatly promote the crystallization behavior, e.g. higher crystallinities. The deep understanding of structure-property relationship expects to guide the synthesis of designed functional polymer materials and the processing of polymer products.

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INTRODUCTION Highly branched polymers (HBPs) are one important subclass of macromolecular structures except for the linear, dendrimer, hyperbranched, and crosslinking polymers.1 Highly branched polymers (HBPs) are highly branched macromolecules with a three-dimensional architecture. The highly branched polymers (HBPs) have garnered significant interest in synthesis and applications because their unique properties differ from linear polymer by ways such as non-entanglement, a large number of end groups and internal cavities, low viscosity, high reactivity and good solubility.2-25 Their internal and peripheral active groups provide a convenient pathway of introducing diverse valuable

functionalities.

Moreover,

highly

branched polymers (HBPs)

display superior

stimuli-responsiveness because of their high density of functionalities. Thus, there are many advanced applications of HBPs in chemical engineering.26 HBPs can be used as a phase forming component for aqueous two-phase systems (ATPS) based on advantages of the lower viscosity, biocompatibility, thermal and chemical stability and a wide range of different functional end-groups.27 Much research has been devoted to designing various advanced materials based on highly branched polymers.28-31 The synthesis of highly branched polymers with precise structure is a great challenge for polymer science and attracts more and more attention. Numerous synthetic pathways for highly branched polymers have been reported. Many kinds of highly branched polymers like star polymers, miktoarm star-shaped polymers and graft polymers were prepared by controlled/living polymerization methods and living free radical polymerization methods,32 such as nitroxide-mediated polymerization (NMP),33 atom transfer radical polymerization (ATRP),34-36 and reversible addition-fragmentation chain transfer (RAFT)37-39 polymerization. “Click” chemistry is an efficient method to create 3



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polymer with new architectures has been intensively applied in the last few years.8 Recently, bio-catalysis has been an interesting complimentary, and the synthesis of highly branched polymers by the combination of enzymatic polymerization with ATRP was investigated.40 Highly branched polymers show similar properties like ideal dendrimers but unlike dendrimers they can by synthesized in one single step resulting in reduced production costs.9, 41-43 However, the methods described above offer little or no control over the molecular weight and the degree of branching (DB) of highly branched polymers. In previous work, we reported a simple method to synthesize linear-comb highly branched poly(ε-caprolactone) with designed molecular weight and well-defined structure.44 In this way, linear-comb branched polymers with a controlled degree of branching and molecular weight were successfully produced. Since then, we continued to synthesize highly branched polymers with more kinds of monomer and more complex architectures and to investigate their properties systematically. Highly branched polymers (HBPs) have obtained tremendous advances because of their wide range of application, especially for biomedical use owing to their superior physical/chemical properties.31 In the field of biomedical polymers, highly branched aliphatic polyesters are an important class of material possessing thermal stability, good mechanical properties, renewability, bio-degradability and bio-assimilability.45-49 Highly branched polymer can be synthesized using a simple “one-step” method, usually by the ring-opening polymerization (ROP) of cyclic esters.50, 51 The common cyclic ester monomers are lactide, glycolide, caprolactone, valerolactone and trimethylene carbonate as well as other, more esoteric cyclic esters. Poly(trimethylene carbonate), PTMC, is an amorphous elastomeric material with good cell permeability and predominantly biomedical application, such as pH-sensitive hydrogel,52 thin composite films,53 and multi-molecular 4


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micelle and drug delivery system.54 Poly(ε-caprolactone), PCL, is a crystalline polyester with low melting point (Tm) and glass transition temperature (Tg) which has significant application in drug delivery systems, in cell cultivation, and in implants for regenerative medicine and drug release.55 PCL can be synthesized using a variety of cationic, anionic and co-ordination catalysts by ring-opening polymerization of ε-caprolactone. Moreover, the degradation rates of PCL and PTMC are lower than that of many other biopolyesters, which would be advantageous in biomedical applications where relatively higher stability is desired. The introduction of highly branched structures would significantly change the properties and applications of traditional materials.2 For example, aliphatic biodegradable star polymers have great advantages in drug delivery systems and nanotechnology applications.56 Amorphous PTMC and crystalline PCL stand for two kinds of materials possessing different aggregation structures, and they have typical physical and chemical properties of amorphous and crystalline polymers. As we know, the influence and regularity of topological structures on the properties of amorphous and crystalline polymers are unique and cannot be discussed lumped together. Considering this, we take PTMC and PCL as the representative polymers to study the influence of structures on the properties of amorphous and crystalline polymers, respectively. Branching architecture greatly influences the physical and chemical properties of the highly branched polymers, including the solution behavior, melting behavior, crystallization and melting behaviors, thermal degradations, glass transition, biomedical applications, and so on.1 Compared to linear polymers, highly branched polymers often exhibit good solubility, lower viscosity, a large number of end functional-groups and smaller hydrodynamic volume.57 Consequently, the highly branched structure can improve the performance and processability of the final product. For example, 5



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highly branched polymer materials with larger molecular weight would be successfully processed by standard industrial equipment due to the lower melting viscosity.58, 59 Commonly studied branched architectures include the simplest branched structure, star polymers, and increasingly complex architectures such as miktoarm stars, graft/comb polymers, H-shaped polymers, and dendritically branched polymers. Among a variety of macromolecular structures, comb polymer is a special class of branched macromolecules in which the side chains are distributed in a specific segment of the macromolecule. In its relative simple structure, comb-like polymers are usually formed by long side chains unlike very short and dense branches of hyper-branched or dendrimer polymers, which make it exhibit different properties from hyper-branched or dendrimer polymers. Predominantly, studies have been aimed at the synthesis and characterization of star-shaped,60-64 hyper-branched,9, 26, 27 and dendrimer polymers.65-67 Nevertheless, a few scientific literatures research the comb-like biodegradable polymers, e.g., Ninago68 reported the influence of branches on the thermal behavior of grafted block copolymers based on ε-caprolacone. Even more, we only found one report about the star-comb branched biodegradable polymers other than the works of our group, e.g., Szoka69 described a six-step synthesis to biodegradable star-comb PEGylated polymers with favorable pharmaceutical properties by ATRP. Moreover, comprehensive comparison on the differences of physicochemical property between comb branched polymers and linear polymers is still scarce and the relationship between architecture and property of highly branched polymers is less clear. Based on this, the synthesis of comb-like polymers and understanding the impact of architectures upon the properties of highly branched polymers have important direct significance for the further development of polymer science. The main content of this research was to assess the influence of topological structure on the 6


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physical and chemical properties of linear and highly branched (star, linear-comb and star-comb) poly(trimethylene carbonate)s and poly(ε-caprolactone)s, respectively. In the current work, we synthesized

well-defined

linear-comb

and

star-comb

poly(trimethylene

carbonate)s

and

poly(ε-caprolactone)s with highly branched architecture and high molecular weight by a simple method using hydroxylated polybutadiene as macroinitiator. For each monomer, linear and star homopolymers were also prepared used as comparison. That is, poly(trimethylene carbonate)s and poly(ε-caprolactone)s with different molecular weight and four kinds of architectures (linear, star, linear-comb and star-comb) were synthesized (Scheme 1). We not only analyzed their structures and characterized their physical and chemical properties systematically including thermal properties, crystal properties, solution viscosities, rheological properties and creep properties, but also discussed the differences in properties of PTMCs and PCLs corresponding to different topological architectures. In particular, we explored systematically for the first time the impact of long-chain, highly branched architecture on the properties of amorphous and crystalline polymers.

Scheme 1. Synthesis of linear, star, linear-comb and star-comb polyesters (PTMC and PCL). 7



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METHODS Materials. Butadiene, n-BuLi, cyclohexane, 2-propanol, tetrachlorosilane were described in our previous publication.70 Formic acid (HCOOH, 98 wt%), hydrogen peroxide (H2O2, 30 wt%) and trifluoromethanesulfomic acid (TfOH, 98%, Aladdin) were available commercially and used as received. Tetrahydrofuran (THF) was distilled from sodium-benzophenone and stored in a flask. Trimethylene carbonate (TMC, 1,3-dioxane-2-one) purchased from JiNan DaiGang was dissolved in THF at a concentration of 0.7 mg/mL and was stirred over CaH2 for 2 days, recrystallized twice from cold THF, finally dried for 24 h. ε-Caprolactone (ε-CL, Aldrich, 99%) was dried over CaH2 and purified

by

vacuum

distillation.

Stannous

octanoate

[Sn(Oct)2,

Aldrich,

97%]

and

1,5,7-Triazabicyclo-[4.4.0]dec-5-ene (TBD, Aldrich, 98%) were used as received. Benzyl alcohol (BnOH) from Aladdin was dried over CaH2 and purified by vacuum distillation. Pentaerythritol (Aldrich) was used as received and kept over P2O5 under vacuum. Dichloromethane, toluene and other solvents were dried and purified in the conventional methods. Characterization. 1H NMR spectra were obtained on a Bruker Avance 400 MHz spectrometer at ambient temperature with CDCl3 as solvent. The number-average molecular weight and dispersity of polymers were determined using GPC (Viscotek TDA-305) in THF at 35 °C calibrated against polystyrene standards. Thermal properties were measured with a differential scanning calorimeter (TA Q2000) on 5-10 mg samples as follows: for PTMC, the test temperature is from -70 °C to 80 °C, and the heating and cooling rates are 10 °C/min; for PCL, the test temperature is from -10 °C to 100 °C, and the heating and cooling rates are 10 °C/min. Thermal degradation properties were characterized by TGA (TA Q500) from ambient temperature to 600 °C at a heating rate of 10 °C/min under nitrogen atmosphere. The intrinsic viscosities ([η]br) of polymers were determined in THF with 8


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a Schott Ubbelohde viscometer at 30.5 ± 0.1 °C and 25.0 ± 0.1 °C for PTMC and PCL, respectively. Creep tests were conducted in tensile mode under room temperature at 0.15 MPa for 10 min using a dynamic mechanical analyzer (TA, DMA Q800). The size of specimens for creep test was as that of DMA tests. Rheological measurements were performed on a stress-controlled Rheometer AR2000 (TA Instruments Ltd) equipped with parallel-plate geometry (diameter of 25 mm and a gap of 1 mm) at constant temperature 70 °C. Dynamic frequency sweep measurements were carried out in an oscillatory shear mode from 100 to 0.1 rad s-1. Wide-angle X-ray diffraction (WAXD) patterns were characterized with X-ray diffractometer (XD-3A). The scanning angle (2θ) covered a range between 5 and 40o at a rate of 5o/min. The samples used for test were prepared at room temperature. Synthesis of Linear and Star Hydroxylated Polybutadienes. The detailed polymerization processes of linear and star hydroxylated polybutadienes were described in our previous publication (Scheme S1).44, 71 Synthesis of Poly(trimethylene carbonate). Synthesis of Linear-comb Highly Branched Poly(trimethylene carbonate) (LC-PTMC). TMC, the macroinitiator (linear hydroxylated polybutadiene, L-HPB) solution and Sn(Oct)2 were transferred into a thoroughly dried glass flask with a magnetic stirring bar. The vessel was degassed by several vacuum-purge cycles that also removed the solvent introduced in the macroinitiator solution. The flask was then sealed under argon placed in an oil bath and held at 130 °C for a predetermined reaction time. After the reaction, the contents of the flask were dissolved in dichloromethane, and then poured into methanol to precipitate the polymer. The volatiles were removed in vacuum. 1H NMR (CDCl3): δ 2.04 (2H, –COOCH2CH2CH2O–), 3.73 (2H, –COO(CH2)2CH2OH), 4.23 (4H, –COOCH2CH2CH2O–). 9



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The synthetic processes of star-comb poly(trimethylene carbonate) (SC-PTMC), linear poly(trimethylene carbonate) (L-PTMC) and star poly(trimethylene carbonate) (S-PTMC) were similar to the synthesis of LC-PTMC except that star hydroxylated polybutadiene (S-HPB) and benzyl alcohol and pentaerythritol were used as initiator instead of L-HPB. Synthesis of Poly(ε-caprolactone) Synthesis of Linear-comb Highly Branched Poly(ε-caprolactone) (LC-PCL). ε-CL was added to a solution of TBD and L-HPB in toluene. The solution was stirred for 10 h and quenched by addition of benzoic acid. The product was isolated by precipitation in cold methanol and then purified by dissolving in toluene and precipitating through slow addition of cold methanol. After filtration, the products were dried under vacuum for 24 h and then were analyzed. 1H NMR (CDCl3): δ 5.41 (CH2CH=CHCH2 PB backbone chain), 4.96 (CH2=CH PB backbone chain), 4.06 (m, C(=O)OCH2 PCL side chain), 3.65 (t, CH2OH), 2.31 (m, CH2C(=O)O PCL side chain), 2.02 (t, CH2CH2 PB main chain), 1.64 (t, CH2CH2CH2 PCL side chain), 1.38 (t, CH2CH2CH2 PCL side chain). Synthesis of Star Poly(ε-caprolactone) (S-PCL). ε-CL, pentaerythritol and Sn(Oct)2 were transferred into a thoroughly dried glass flask with a magnetic stirring bar. The vessel was degassed by several vacuum-purge cycles. The flask was then sealed under argon placed in an oil bath and held at 130 °C for a predetermined reaction time. After the reaction, the contents of the flask were dissolved in dichloromethane, and then poured into methanol to precipitate the polymer. The volatiles were removed in vacuum. The

synthetic

processes

of

star-comb

poly(ε-caprolactone)

(SC-PCL)

and

linear

poly(ε-caprolactone) (L-PCL) were similar to the synthesis of LC-PCL except that star hydroxylated polybutadiene (S-HPB) and benzyl alcohol were used as initiator instead of L-HPB. 10


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RESULTS AND DISCUSSION Synthesis and Characterization of Macro-initiator. Four linear macro-initiators with different epoxidations and two star macro-initiators were prepared (Table 1) and used for the synthesis of linear-comb and star-comb poly(trimethylene carbonate)s and poly(ε-caprolactone)s with different degrees of branching. The characterization (Figure S1 and S2) and calculation of E please refer the supporting information and the previous work72. Table 1. Molecular Weight and Degree of Epoxidation of Macroinitiators Mwa (Da) Ða Eb (mol%) sample Mna (Da) L-HPB1 4400 5000 1.14 16.4 L-HPB2 4600 5200 1.14 19.6 L-HPB3 4900 5600 1.14 27.6 L-HPB4 6100 6600 1.08 23.7 S-HPB1 12000 13300 1.11 25.2 S-HPB2 12400 13800 1.11 12.3 a b 1 Measured by GPC in THF. Degree of epoxidation. Measured by H NMR in CDCl3.

Synthesis of Highly Branched Poly(trimethylene carbonate)s (Linear-comb and Star-comb). The linear-comb and star-comb poly(trimethylene carbonate)s were synthesized by simple “one-step” and “graft from” strategies using hydroxylated polybutadiene as macroinitiator and Sn(Oct)2 as catalyst (Scheme 1). The results were listed in Table 2. Two series of LC-PTMCs and SC-PTMCs with different molecular weight were prepared by changing the monomer feed ratio with hydroxyl groups. It can be seen that the yield of polymers was very high near quantitative conversion. Significantly, the weight-average molecular weights for linear-comb PTMC and star-comb PTMC reach up to 364 kg/mol and 270 kg/mol, respectively. For the sake of comparison, two other series of linear and star shaped PTMCs with different molecular weight were also synthesized (Table 2).

11



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Table 2. The Results of Poly(trimethylene carbonate)s with Different Structures Synthesized by Sn(Oct)2a Mnb [η]brd Strainmax Mwb [η]line sample Ðb Yieldc(%) g'f -1 -1 (KDa) (ml g ) (ml g ) (KDa) (%) g L-PTMC1 39 80 2.05 97.8 -57.55 -112 L-PTMC2 51 87 1.70 100 -61.03 -38 L-PTMC3 110 226 2.05 100 -116.74 -15 L-PTMC4 217 406 1.87 98.2 -173.56 -5 h S-PTMC1 76 121 1.59 100 51.98 76.48 0.68 119 S-PTMC2 87 137 1.58 100 57.54 83.18 0.69 47 S-PTMC3 92 166 1.80 100 58.50 94.73 0.62 46 S-PTMC4 115 169 1.47 100 62.96 95.89 0.66 15 i LC-PTMC1 63 155 2.46 98.1 58.85 90.43 0.65 31 LC-PTMC2 64 237 3.70 98.4 57.31 120.55 0.48 77 LC-PTMC3 62 197 3.16 88.0 59.29 106.37 0.56 44 0.50 19 LC-PTMC4 119 364 3.06 99.1 80.33 161.19 LC-PTMC5 139 267 1.92 98.2 72.98 130.69 0.56 18 j SC-PTMC1 97 142 1.47 95.6 48.98 85.23 0.57 103 SC-PTMC2 103 205 1.99 95.2 53.70 109.30 0.49 115 SC-PTMC3 116 223 1.92 97.4 56.62 115.69 0.49 96 SC-PTMC4 140 270 1.93 97.0 60.25 131.68 0.46 119 a Reaction temperature is 130 °C, in bulk, 24h, [Sn(Oct)2]/[M] (mol) = 0.001. b Measured by GPC in THF. c Calculated by the weight of monomer and product. d [η]br was obtained in THF at 30.5 ± 0.1 °C. e [η]lin was calculated using [η]lin = KMwα; K = 0.0277 mL/g, α = 0.677. f g’ = [η]br /[η]lin. g L-PTMC means linear poly(trimethylene carbonate). h S-PTMC means star poly(trimethylene carbonate), initiated by pentaerythritol. i LC-PTMC means linear-comb poly(trimethylene carbonate). LC-PTMC1 initiated by L-HPB3, LC-PTMC2 initiated by L-HPB1, other samples initiated by L-HPB2. j SC-PTMC means star-comb poly(trimethylene carbonate), initiated by S-HPB1. The 1H NMR spectrum of highly branched poly(trimethylene carbonate) was shown in Figure S3. The position of each characteristic proton peak is conformed with the structure of PTMC describing in the literature.73 Peaks at 2.04 and 4.23 ppm are assigned to PTMC repeat units. The peak at 3.73 ppm is assigned to the methylene protons adjacent to the end hydroxyl group. The molecular weight and dispersity of linear and highly branched poly(trimethylene carbonate)s were further characterized by GPC analysis (Figure S4). The GPC curves of all polymers with different 12


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architectures showed a unimodal peak, which indicated that the polymerization process was performed under good control. Synthesis of Highly Branched Poly(ε-caprolactone)s (Linear-comb and Star-comb). The linear-comb and star-comb poly(ε-caprolactone)s were synthesized by simple “one-step” and “graft from” strategies initiated by hydroxylated polybutadiene and catalyzed by TBD as Scheme 1 and the results are listed in Table 3. Two series of LC-PCLs and SC-PCLs with different molecular weight were synthesized by controlling the feed ratio of monomer with hydroxyl groups. It can be seen that the yield of polymers was very high. Significantly, the weight-average molecular weight for linear-comb PCL reaches up to 276 kg/mol. For the sake of comparison, two other series of linear and star shaped PCLs were also synthesized (Table 3). Table 3. The Results of Poly(ε-caprolactone)s with Different Structures Synthesized by TBD or Sn(Oct)2a b b Mn Mw Ðb sample Yieldc(%) [η]brd (ml g-1) [η]line (ml g-1) g'f (KDa) (KDa) g L-PCL1 17 25 1.45 96.9 -35.27 -L-PCL2 21 30 1.44 98.5 -40.15 -L-PCL3 45 63 1.40 94.5 -67.99 -L-PCL4 53 72 1.35 89.8 -74.75 -h S-PCL1 25 40 1.59 95.8 31.33 49.24 0.64 S-PCL2 37 64 1.74 94.4 44.96 68.75 0.65 S-PCL3 45 78 1.74 96.9 52.50 79.12 0.66 S-PCL4 52 90 1.73 98.0 53.42 87.58 0.61 i LC-PCL1 84 95 1.13 92.4 29.11 91.01 0.32 LC-PCL2 150 167 1.11 93.5 36.67 135.83 0.27 LC-PCL3 190 213 1.12 93.4 43.04 161.45 0.27 LC-PCL4 251 276 1.10 94.1 54.34 194.06 0.28 j SC-PCL1 47 61 1.29 91.8 17.94 66.45 0.27 SC-PCL2 87 102 1.17 95.5 26.03 95.72 0.27 SC-PCL3 107 121 1.13 91.5 27.08 108.06 0.25 SC-PCL4 152 175 1.15 92.5 34.04 140.42 0.24 a Reaction conditions: [TBD]/[M] (mol) = 0.01,CM = 2.0 mol/L, 13h, toluene as solvent, 20 °C. [Sn(Oct)2]/[M] (mol) = 0.001, 130 °C, in bulk, 24h. b Measured by GPC in THF. 13



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c

Calculated by the weight of monomer and product. [η]br was obtained in THF at 25.0 ± 0.1 °C. e [η]lin was calculated using [η]lin = KMwα; K = 0.0266 mL/g, α = 0.71. f g’ = [η]br /[η]lin. g L-PCL means linear poly(ε-caprolactone), catalyzed by TBD. h S-PCL means star poly(ε-caprolactone), initiated by pentaerythritol and catalyzed by Sn(Oct)2. i LC-PCL means linear-comb poly(ε-caprolactone), initiated by L-HPB4 and catalyzed by TBD. j SC-PCL means star-comb poly(ε-caprolactone), initiated by S-HPB2 and catalyzed by TBD. d

The 1H NMR spectrum of highly branched poly(ε-caprolactone) was shown in Figure S5. The position of each characteristic proton peak is conformed with the structure of PCL describing in the literature.74 The peaks at 2.02 ppm and 4.96, 5.41 ppm are assigned to the methylene groups and methyne groups on the PB main chain. The peaks at 1.38, 1.64, 2.31 and 4.06 ppm are assigned to PCL repeat units and the peak at 3.65 ppm is assigned to the methylene protons connecting with terminal hydroxyl groups. The molecular weight and dispersity of linear and highly branched poly(ε-caprolactone)s were further characterized by GPC analysis (Figure S6). The GPC curves of all polymers with different architectures showed a unimodal peak, which indicated that the polymerization process was performed under good control. The highly branched polymers (HBPs) have obtained enormous development in the synthesis and applications due to their unique properties different from linear polymer.9, 41, 42, 75 This part of the article would focus on the discussion of the molecular properties of the highly branched poly(trimethylene carbonate) and poly(ε-caprolactone) aimed to obtain deep understanding about the structure-property relationship. Intrinsic Viscosity. The solution behaviors of highly branched poly(trimethylene carbonate) and poly(ε-caprolactone) were investigated by intrinsic viscosities measurement. For a given solvent system and specified temperature, the relationship of intrinsic viscosity ([η]) and molecular weight of polymer is conformed to the Mark-Houwink equation:76 14


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[η ] = KM wα

(1)

Where K and α are Mark-Houwink coefficients, and Mw is the weight-average molecular weight. [η]br means the intrinsic viscosity of highly branched polymers and was measured by Schott Ubbelohde viscometer. [η]lin means the intrinsic viscosity of their linear analogs with the same molecular weight and was calculated using Mark-Houwink equation. Then the branching factor (g’) can be calculated by the following equation: g'=

[η ]br [η ]lin

(2)

The intrinsic viscosities of linear, star, linear-comb and star-comb poly(trimethylene carbonate)s ([η]br and [η]lin) determined by Ubbelohde viscometer and branching factor (g’) were summarized in Table 2. Obviously, the values of experimental [η]br for highly branched poly(trimethylene carbonate)s (star, linear-comb and star-comb) were significantly lower than theoretical [η]lin for their linear analogs with the same molecular weight. In addition, it is also observed that the values of g’ are in an order of L-PTMCs > S-PTMCs > LC-PTMCs > SC-PTMCs. The branching factor (g’) reflects the degree of branching (DB) of polymer, and the smaller the branching factor, the higher the degree of branching. Therefore, the degree of branching is in an order of SC-PTMCs > LC-PTMCs > S-PTMCs > L-PTMCs. A lower branching factor (g’) suggested that the highly branched poly(trimethylene carbonate) behaved more rigid (denser) sphere-like behavior. Figure 1 shows the logarithmic plot of intrinsic viscosities versus molecular weight of poly(trimethylene carbonate)s with four structures (linear, star, linear-comb and star-comb). Appreciably, under the condition of the same molecular weight, the intrinsic viscosities of L-PTMCs were the highest, followed by S-PTMCs, and the intrinsic viscosities of SC-PTMCs were the smallest. It can be concluded that the intrinsic viscosities decreased with increasing the degree of 15



branching arise from the branching architecture. In THF, linear PTMCs behave more unconsolidated, highly branched PTMCs exhibit more compact structure and smaller hydrodynamic volumes.77

The

data in the logarithmic plot (Figure 1) was linear fitted, and the straight line represents the equation: ln[η ] = ln K + α ln( M w )

(3)

where K = 0.0277 mL/g, α = 0.677. Typically, the Mark-Houwink exponent α reflects the shape and compactness of a polymer in a given solvent. Many studies indicate that the value of α ranges between 0.3 and 0.5 for highly branched polymers usually,78 whereas the value of α ranges between 0.5 and 1 for linear polymers typically. The Mark-Houwink coefficients for linear and highly branched PTMCs were calculated by equation (3), shown in Figure 1. As expected, the α values for highly branched PTMCs were smaller than linear PTMCs. It is in an order of L-PTMCs > S-PTMCs > LC-PTMCs > SC-PTMCs, which is in line with the order of increasing chain compactness.

2.3 2.2 2.1 log[η]

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 41

L-PTMC S-PTMC LC-PTMC SC-PTMC α = 0.68

2.0 1.9 α = 0.52

1.8

α = 0.47 α = 0.32

1.7 4.8

4.9

5.0

5.1

5.2

5.3 logMw

5.4

5.5

5.6

5.7

Figure 1. Intrinsic viscosities vs. molecular weights of PTMCs with different structures in THF at 30.5 °C.

16


Page 17 of 41

2.0 L-PCL S-PCL LC-PCL SC-PCL

1.9 1.8

α = 0.71

α = 0.68

1.7 log[η]

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


α = 0.57

1.6 1.5

α = 0.44

1.4 1.3 1.2 4.4 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 log Mw

Figure 2. Intrinsic viscosities vs. molecular weights of PCLs with different structures in THF at 25.0 °C.

The intrinsic viscosities of linear, star, linear-comb and star-comb poly(ε-caprolactone)s ([η]br and [η]lin) determined by Ubbelohde viscometer and branching factor (g’) were summarized in Table 3. Obviously, the values of experimental [η]br for highly branched poly(ε-caprolactone)s (star, linear-comb and star-comb) are significantly lower than theoretical [η]lin for their linear analogs with the same molecular weight. In addition, the degree of branching is in an order of SC-PCLs > LC-PCLs > S-PCLs > L-PCLs. Figure 2 shows the intrinsic viscosities vs molecular weight for linear and branched PCLs. Highly branched PCL showed a significantly lower [η] than linear PCL, even though their molecular weights were much higher than those of the linear analogues, confirming the branched structure. In THF solution, the highly branched poly(ε-caprolactone) behaved more compact structure and exhibit smaller hydrodynamic volumes. The intrinsic viscosity decreases with the degree of branching increasing. The Mark-Houwink coefficients for linear and highly branched PCLs were calculated by equation (3), shown in Figure 2. As a result, the α values for highly branched PCLs were smaller than linear PCLs. It is in an order of L-PCLs > S-PCLs > LC-PCLs > SC-PCLs, which is in line with the order 17



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 41

of increasing chain compactness. It reflects the fact that the intrinsic viscosities of all polymers increase with increasing molecular weight, whereas it is slower for highly branched polymers than the linear ones.79 80, 81

Thermal

Properties

of

Amorphous

Poly(trimethylene

carbonate)s.

Glass-transition

temperature (Tg) is one of the typical characteristics of polymers. Glass-transition temperature (Tg) is significantly influenced by the mobility of the polymer backbone decided by spatial structure and chemical composition. In addition, the Tg reflects the free volume of the molecules which depends on the end group numbers and interactions. The thermal parameters were listed in Table 4 and the DSC curves of linear, star, linear-comb and star-comb PTMC with similar molecular weight were shown in Figure 3. It can be seen that direct comparison of linear, star, linear-comb and star-comb PTMCs having identical repeating unit chemistry showed practically identical glass-transition temperatures. For a traditional linear polymer, the glass transition originates from the long-range segmental motions. However, for a highly branched polymer, the glass transition is associated with many co-operative interactions. With increasing degree of branching (DB), the distribution of the molecular structure in space becomes more compact leading to the mobility of molecular chain decreasing. On the other hand, the free volume increases with increasing degree of branching contributed from the terminal units, which strengthens the molecular mobility.82 The end group effect on Tg in highly branched systems is very strong as well. For poly(trimethylene carbonate), H-bonding interactions increase with increasing DB and the number of end group.57 Thus, Tg with different architectures exhibiting similar values might be related to the changes of free volume and interacting end-groups simultaneously.

18


Page 19 of 41

L-PTMC3 S-PTMC4 LC-PTMC4 SC-PTMC3 o

-12.6 C

Heat flow (exo up)

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


o

-13.3 C o

-14.1 C o

-13.9 C

-40

-30

-20

-10

0

10

20

o

Temperature( C)

Figure 3. DSC melting curves of PTMCs with different structures. Thermal stability is one of the important properties of polymer determining their use in many applications.83-85 Thermal degradations of PTMCs with different structures were also analyzed as shown in Table 4 and Figure S7. Td and Td,max of PTMCs from linear to highly branched have no visible difference. This fact shows that Td is primarily determined by the chemical composition of polymer.

sample L-PTMC1 L-PTMC2 L-PTMC3 L-PTMC4 S-PTMC1 S-PTMC2 S-PTMC3 S-PTMC4 LC-PTMC1 LC-PTMC2 LC-PTMC3 LC-PTMC4 LC-PTMC5 SC-PTMC1 SC-PTMC2

Table 4. Thermal Parameters of PTMCs with Different Structures Mna (KDa) Mwa (KDa) g' Tg (°C) Td,5% (°C) Td,95% (°C) 39 80 --12.7 276.2 312.8 51 87 --12.2 261.2 304.1 110 226 --12.6 273.5 312.8 217 406 --13.8 264.4 298.8 76 121 0.68 -15.2 282.1 307.5 87 137 0.69 -14.8 279.3 306.8 92 166 0.62 -15.5 285.7 311.7 115 169 0.66 -13.3 284.1 307.7 63 155 0.65 -13.5 285.1 310.2 64 237 0.48 -14.0 263.1 302.6 62 197 0.56 -13.2 281.9 305.8 119 364 0.50 -14.1 284.4 307.3 139 267 0.56 -13.5 287.5 309.7 97 142 0.57 -15.9 282.3 308.1 103 205 0.49 -12.9 283.4 310.4

Td,max (°C) 303.0 294.3 305.0 292.1 301.3 302.1 305.8 303.9 305.0 297.7 297.3 302.7 305.4 303.2 304.8 19



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

SC-PTMC3 116 SC-PTMC4 140 a Measured by GPC in THF.

223 270

0.49 0.46

-13.9 -14.4

289.8 283.8

Page 20 of 41

311.6 308.4

305.8 303.6

Creep Properties of Amorphous Poly(trimethylene carbonate)s. A distinct advantage of highly branched polymers is their preferable melt rheological behavior, which is different from that of linear polymers and makes it easier to process and inject.43 In order to take advantage of highly branched polymers for the practical production, it is necessary to deeply understand the impact of branched architecture on the melting behavior of highly branched polymers. Poly(trimethylene carbonate) is amorphous elastic rubber with low Tg, and there is no viscous flow state. Thus, studies on the creep performance of PTMCs with various architectures were carried out. Creep is an important time-dependent mechanical property of polymers determining the durability and reliability of materials.86, 87 Creep experiment studies the viscoelasticity of materials, and creep behavior is the result of the interaction of elastic and viscous. Usually, materials used for processing require good creep property (excellent mobility), and materials used for practical application require poor creep performance (good creep resistance). Given that the creep strain of polymer are sensitive to the temperature changes,88, 89 creep test temperature was set as 25 °C. The typical creep strain data of each kind of PTMCs were summarized in Table 2. Figure 4 displays the creep strain curves as a function of time for PTMCs. In these curves, the creep stages (instantaneous deformation, primary and secondary creeps) can be clearly observed. It is visibly apparent that the creep strain decreased with the increasing of molecular weight for each kind of structure of PTMCs. Aliphatic polycarbonate is an elastic rubber material. Because rubber is a long-chain molecule, the motion of entire molecule or segment must overcome intermolecular forces and internal friction, and high-elastic deformation is achieved by the motion of molecule segments. At room temperature, 20


Page 21 of 41

long-chain molecules of polycarbonate are in twist state and extend along the direction of external force inevitably under constant external force, showing obvious creep behavior. With increasing molecular weight, the internal friction increases during the molecule segment movement. Furthermore, longer chain makes it easier for polycarbonate molecule to entangle. Hence, PTMCs exhibited that the creep strain and creep rate decreased with the increasing of molecular weight. (a)

120 L-PTMC1 39000 L-PTMC2 51000 L-PTMC3 110000 L-PTMC4 217000

Creep strain (%)

100 80 60 40 20

S-PTMC1 76000/0.68 S-PTMC2 87000/0.69 S-PTMC3 92000/0.62 S-PTMC4 115000/0.66

80 60 40 20

0 6

8

10 12 Time(min)

14

0

16

6

8

10 12 Time (min)

14

16

120 (d)

(c) LC-PTMC2 64000/0.48 LC-PTMC3 62000/0.56 LC-PTMC1 63000/0.65 LC-PTMC4 119000/0.50 LC-PTMC5 139000/0.56

60

100 Creep strain (%)

80

(b)

100 Creep strain (%)

120

Creep strain (%)

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


40

20

80 60 40 SC-PTMC1 97000/0.57 SC-PTMC2 103000/0.49 SC-PTMC3 116000/0.49 SC-PTMC4 140000/0.46

20 0

0 6

8

10 12 Time (min)

14

16

6

8

10 12 Time (min)

14

16

Figure 4. The relationship of creep strain and time of PTMCs with different structures. 0.15MPa stress for 10min. (a) linear PTMC, (b) star PTMC, (c) linear-comb PTMC, (d) star-comb PTMC.

21



100

L-PTMC3 110000 S-PTMC4 115000/0.65 LC-PTMC4 119000/0.50 SC-PTMC3 116000/0.49

80 Creep strain (%)

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 22 of 41

60

40

20

0 6

8

10 12 Time(min)

14

16

Figure 5. The relationship of creep strain and time of PTMCs with different structures. 0.15MPa stress for 10 min. For samples LC-PTMC (1-3) with similar molecular weight, it can be seen that the creep strain and rate increased with decreasing branching factor (g’, increasing degree of branching). Thus, the creep behavior toward the degree of branching change is very sensitive. Furthermore, we compared the creep properties of PTMCs with different structures. The results shown in Figure 5 can also indicate that creep strain of linear PTMC was lower than that of branched PTMCs. It is obviously that the creep strain and rate increased remarkably with degree of branching increasing, and for example, at similar molecular weight the creep strains of L-PTMC and SC-PTMC are 15% and 96%, respectively. For sample SC-PTMC3, the creep strain of material increased sharply as time increasing which has reached the tertiary creep, i.e. creep rupture. This is related to the very high DB and short molecular chains, which make it difficult for the highly branched molecules to entangle. Besides, the distance between molecules increases with the branching density, leading to intermolecular forces decreasing. So the movement of molecule segment becomes easier. It is clearly visible that the degree of branching and topology have a strong influence on the creep properties.

Rheological Properties of Crystalline Poly(ε-caprolactone)s. Besides the solution behavior, the 22


Page 23 of 41

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


melt property of highly branched polymers is one of the major advantages. The melt rheological behaviors of PCLs with different structures were studied by a rheometer at 70 °C, and a strong relation between the DB and melt rheological properties had been observed. Figure S8 showed the complex viscosity (η*), storage modulus (G’), and loss modulus (G’’) for PCLs with four kinds of structures (linear, star, linear-comb and star-comb). As seen in Figure S8, the complex viscosity (η*) of all samples decreased with increasing frequency, exhibiting typical shear thinning. That is an indication of a pseudo plastic behavior of the melt at 70 °C. For each kind structure of PCL, the complex viscosity (η*) increased as the molecular weight increasing. Viscous flow is a process of significant changes in the relative position between the molecules. With increasing molecular weight, the internal friction increases during the molecule motion. Furthermore, the thermal motion of the longer molecular chain hinders the movement of the whole molecule toward a certain direction. So, the movement of the whole molecular chain becomes more difficult with increasing molecular weight, and the complex viscosity (η*) increases. The storage modulus (G’) and the loss modulus (G’’) reflect the elasticity and viscidity of materials, respectively. They revealed similar trends with the complex viscosity corresponding to the molecular weight. The topological structure has significant influence on the rheological properties of polymers.90 A comparison on the rheological properties of PCLs with different structures was made using dynamic oscillation and steady shear measurements. As shown in Figure 6 all branched PCLs, especially linear-comb and star-comb PCL, showed lower zero-shear viscosity than linear PCL. It can be explained by that the shorter molecular chains and more compact structures than linear PCL reduce the volume of the branched PCL. The complex viscosities (η*) of star and comb PCLs were considerably lower than that of linear PCL although their molecular weights were higher. This 23



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 24 of 41

deviation can be explained by the state of molecular chain. Poly(ε-caprolactone) is a flexible polymer, entanglement is easy for linear PCL with long chain and inhibits slippage and reorientation of polymer chain. The very high DB and short molecular chains make it difficult for highly branched poly(ε-caprolactone) to entangle. For this reason, steric hindrance during flow process decreases and complex viscosity also decreases for highly branched poly(ε-caprolactone)s. This can be demonstrated by storage modulus (G’) (Figure S8). For all samples, in the low-frequency region the storage modulus (G’) exhibited similar dependencies according to the dynamic scaling theory based on the Rouse model.91 However, differences appeared in the high-frequency region, where a slight elastic rubber plateau was observed in the case of linear PCLs, typical for polymer chain entanglements (Figure S8-a2). Compared linear-comb LC-PCL2 and star-comb SC-PCL4 with similar molecular weight (Figure 6), unexpected, we found an interesting performance that the complex viscosity (η*) of star-comb PCL was higher than that of linear-comb PCL. It is arised from the stronger interaction of terminal hydroxyl groups for star-comb PCL. The end group has significant influence on the chain mobility and the rheology property of highly branched polymers. For star-comb PCL, more end groups and more closeness of the side chains, the interactions between the polar OH end groups enhanced and the high complex viscosity was achieved. As seen in Figure 6b, at high frequencies, G’ value for SC-PCL4 was higher than that of LC-PCL2. But at low frequencies region, this function in SC-PCL4 was lower than that of LC-PCL2. For many polymers, the slope of G’ at low frequencies is 2.92 The slope of G’ for SC-PCL4 was smaller than the above mentioned value and smaller than that of LC-PCL2, which implys the existence of a significant amount of entanglement.93 Some of researchers suggested that the hydrogen bonds act as temporary entanglements and are in equilibrium 24


Page 25 of 41

between forming and breaking.94 The loss modulus (G") means the energy lost to the viscous deformation during the deformation process of materials. The value of G" reflects the viscosity of materials. The loss modulus (G") decreases with the viscosity of polymer decreasing. Figure 6c shows that the loss modulus (G") of L-PCL3 was the highest, followed by S-PCL2, and the loss modulus (G") of LC-PCL2 was the smallest, which displays similar trends with the complex viscosity corresponding to the structure of PCL. As a consequence, the complex viscosities (η*) is in an order of L-PCLs > S-PCLs > SC-PCLs > LC-PCLs.

3

4

10

3

10

10

2

10

10

1

G' (Pa)

10

2

10

10 1

10 0.01

5

10

10

(b)

1 10 Frequency (Hz)

100

0

10 0.01

3

2

10

L-PCL3 63000 S-PCL2 64000/0.65 LC-PCL2 167000/0.27 SC-PCL4 175000/0.24

-1

0.1

(c)

4

G'' (Pa)


(a)

η*(Pa· s)

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


0.1

1 10 Frequency (Hz)

100


1

10

0

10 0.01

0.1

1 10 Frequency (Hz)

100

Figure 6. Variation in: (a) complex viscosity (η*), (b) storage modulus (G’) and (c) loss modulus (G’’) as a function of frequency for PCLs with different structures.

Crystallization Properties of Crystalline Poly(ε-caprolactone)s. The crystallization of crystalline polymers is closely related to their chain architecture. In order to characterize the crystallizability and crystal structure of PCLs with different topologies, we examined WAXD patterns of linear, star, linear-comb and star-comb PCLs and made a comparison. Firstly, as shown in Figure 7, two sharp crystalline peaks at 2θ = 21o and 24o were observed for each WAXD pattern attributed to the diffraction from (110) and (200) lattice planes of PCL, respectively.

95-97

This indicates that the

branched PCLs crystallized in the same pattern with linear PCL and highly branched architecture had 25



no obvious effect on the crystal structure of PCL.

(110) (200)

SC-PCL

Intensity(a.u.)

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 26 of 41

LC-PCL

S-PCL

L-PCL

10

15

20

25 o 2θ( )

30

35

40

Figure 7. WAXD patterns of PCLs with different structures.

Table 5. Thermal Parameters and Crystallinities of PCLs of Different Structures sample Mwa (KDa) Tc (°C) △Hc (J g-1) Tm (°C) △Hm (J g-1) Xcb(%) L-PCL3 63 31.0 64.8 58.8 62.5 59.8 L-PCL4 72 29.3 61.7 57.6 58.6 58.0 S-PCL3 78 24.3 59.3 56.9 58.5 60.5 S-PCL4 90 26.1 59.8 57.1 59.1 61.7 LC-PCL1 95 28.0 70.1 56.5 68.7 85.5 LC-PCL2 167 26.3 68.9 55.5 68.0 87.6 SC-PCL2 102 32.3 79.4 55.4 77.2 87.8 SC-PCL4 175 37.5 83.3 56.3 80.9 89.5 a Measured by GPC in THF. b Crystallinities were measured by XRD.

26


Page 27 of 41

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


Table 6. Crystallite Sizes of L110 and L200 of PCLs with Different Structures L110 L200 Mwa sample b c b c (KDa) 2θ β dhkl (nm) D (nm) 2θ β dhkl (nm) L-PCL4 72 21.60 0.385 0.41 20.78 23.94 0.486 0.37 S-PCL3 78 21.48 0.420 0.41 19.05 23.84 0.482 0.37 LC-PCL1 95 21.38 0.727 0.42 11.00 23.74 0.675 0.37 21.56 0.700 0.41 11.43 23.88 0.704 0.37 SC-PCL2 102 a Measured by GPC in THF b Bragg angle c Measured half-width of the experimental profile

D (nm) 16.53 16.67 11.90 11.41

The values of crystallinities and thermal performance parameters of PCLs with linear and branched structures were presented in Table 5. It shows that the crystallinities of linear-comb PCLs and star-comb PCLs were much higher than those of linear PCLs and star PCLs. Compared samples L-PCL4, S-PCL4, LC-PCL1 and SC-PCL2 with similar molecular weight, it can be seen that the crystallinity increased with increasing degree of branching (from 58.0% for L-PCl4 to 87.8% for SC-PCL2). This clearly indicates that a remarkable improvement in crystallization behavior was achieved for branched PCL compared with linear one. It is demonstrated by crystallization and melting traces (Figure S9), the crystalline enthalpy (△Hc) and melting enthalpy (△Hm) also increased from linear to branched PCLs. The molecular weights of all polymers we synthesized were very high leading to very long molecular chains, which makes it easy for long chain linear polymers to entangle and inhibit slippage and reorientation of polymer chains. The introduction of branched structure to polymer weakened the entanglement and increased segmental mobility of branched polymers with shorter side chains. When the molecular chain is long enough, star and star-comb PCLs can be viewed as the cross of linear and linear-comb PCLs, respectively. In terms of nature structure, there is no significant difference between linear and star as well as linear-comb and star-comb PCLs. Therefore, the degrees of crystallinity of star and star-comb PCLs were similar to linear and 27



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 28 of 41

linear-comb PCLs, respectively. In a word, the crystallization ability is significantly improved form linear to branched PCL. The crystallite sizes (D) of PCLs were also calculated and shown in Table 6. Apparently, the values of D (L110 and L200) for linear-comb PCLs and star-comb PCLs were significant lower than that of linear PCL. It is related to the very high DB and the dense branching topology resulting in the more nucleating point and higher nucleation density for branched PCL.

CONCLUSIONS In this research, highly branched linear-comb and star-comb poly(trimethylene carbonate)s (PTMC) and poly(ε-caprolactone)s (PCL) with well-defined structure and high molecular weight were successfully synthesized using linear and star hydroxylated polybutadiene (HPB) as macroinitiators by simple “one-step” and “graft from” strategies. Series of comb branched PTMCs and PCLs, with different degree of branching (DB), could be controlled prepared by changing the number of hydroxyl on HPB backbones. Meanwhile, series of linear and star PTMCs and PCLs with different molecular weights were also produced for comparison. PTMC is amorphous polymer and PCL is crystalline polymer, they represent two kinds of materials with different aggregation structures and possess different physical and chemical properties. As known, many properties of highly branched polymers, including the solution behavior, rheological property, crystallization property, glass transition and thermostability are unique when compared to linear polymer and are highly related with the branching structures. The critical content of this research focused on the characterization and comparison of properties for PTMCs and PCLs with different topologies and for the first time we comprehensively constructed the structure-property relationship of highly branched amorphous polymers and crystalline polymers. For amorphous PTMC, (1) the intrinsic viscosities decreased with 28


Page 29 of 41

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


increasing degree of branching arise from more compact structure and smaller hydrodynamic volumes for comb branched PTMC, (2) the creep strain and rate increased remarkably with degree of branching increasing due to the very high DB and the shorter side chains making it difficult for the highly branched molecules to entangle, (3) the glass transitions and thermal degradations had no significant difference for PTMCs with different structures because they are primary determined by the chemical composition of polymer. For crystalline PCL, (1) intrinsic viscosity measurement confirmed that the comb branched PCLs displays much lower intrinsic viscosities than linear one and the star-comb PCL showed the smallest hydrodynamic volumes in solution due to dense branching structure, (2) rheological measurement demonstrated the shear thinning behavior for all polymers and the complex viscosities for comb branched PCLs were much lower than that of linear PCL because of less entanglement, (3) both WAXD and DSC analysis indicated that the comb branched architectures have no significant influence on the crystal structure of PCL, but greatly promote the crystallization behavior, e.g. higher crystallinities. The deep understanding of structure-property relationship expects to guide the synthesis of designed functional polymer materials and open up potential application areas.

ACKNOWLEDGMENTS This work was financially supported by National Program on Key Basic Research Program of China (973 Program No. 2015CB654700 (2015CB654701)) and National Science Foundation of China (No. U1508204).

Supporting Information Available: Synthetic routes, 1H NMR spectra and GPC curves of linear and star hydroxylated polybutadiene; 1H NMR spectra and GPC curves of PTMC and PCL; TGA and DTG curves of PTMC; rheological properties and DSC curves of PCL. 29



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40


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TOC IMAGE Relationships Between Architectures and Properties of Highly Branched Polymers: The Cases of Amorphous Poly(trimethylene carbonate) and Crystalline Poly(ε-caprolactone) Yingying Ren, Zhiyong Wei, Xuefei Leng, Tong Wu, Yufei Bian, and Yang Li∗

O

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O

O

nH

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O

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ct) 2 (O Sn 0o C 13

OH + O

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Star PTMC (PCL)

O or O CL

O

S-HPB

TMC

L-HPB

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


Srar-comb PTMC (PCL)

PB x

n

m-x

O PTMC

O

On

O Linear-comb PTMC (PCL)

∗

PCL

O n

Corresponding author.

E-mail address: [email protected] (Y. Li). Tel.: +86-411-84981006 41


Relationships between Architectures and Properties of Highly

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