Multiple Segmental Processes in Polymers with cis and trans

Dec 14, 2017 - Altering the stereochemistry of a single double bond in the side group of a polymer resulted in systems with unprecedented local dynami...
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Letter Cite This: ACS Macro Lett. 2018, 7, 11−15

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Multiple Segmental Processes in Polymers with cis and trans Stereoregular Configurations Achillefs Pipertzis,† Andreas Hess,‡ Philipp Weis,‡ George Papamokos,† Kaloian Koynov,‡ Si Wu,*,‡ and George Floudas*,† †

Department of Physics, University of Ioannina, 45110 Ioannina, Greece Max Planck Institute for Polymer Research, 55128 Mainz, Germany

‡

S Supporting Information *

ABSTRACT: Altering the stereochemistry of a single double bond in the side group of a polymer resulted in systems with unprecedented local dynamics. These include (i) the appearance of three segmental processes in the cispolymers all with Vogel−Fulcher−Tammann (VFT) temperature dependence, (ii) the low steepness index associated with fragility, m, and (iii) the lowest pressure coefficient of Tg, dTg/dP, ever reported for polymers. We show that it is the inability of the cis-polymer to pack the side groups efficiently that controls the dynamics. Furthermore, the trans-polymers have the ability to crystallize. The wealth of dynamics reflects the cis/trans stereochemistry and the presence of different dipoles at specific positions sampling both the side group and backbone dynamics.

W

azopolymers are relatively unstable with respect to temperature and time. Here we report the synthesis, the self-assembly, and the dynamics of polymers bearing side groups with cis and trans unsaturated fatty acids. We show that the resulting cis- and trans-polymers exhibit features that are not common to homopolymers with double bond stereochemistry in their backbone. Except for the ability of the trans-polymer to crystallize, a common feature in both classes of polymers, differences include the presence of three segmental processes, the very low fragility index and the lowest pressure coefficient of Tg ever reported for polymers. For the cis-polymer these unique features reflect the inability of side groups to pack efficiently. In essence, polymers with cis and trans configurations in their side group supersede nanophase separated poly(n-alkyl methacrylates)1−4 and polymers where these configurations are incorporated in the backbone7 with respect to the wealth of dynamic processes. The cis- and trans-unsaturated fatty acid containing polymers were synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization and fully characterized by 1H nuclear magnetic resonance (NMR) and 13C NMR spectroscopy and gel permeation chromatography (GPC).12,13 Details on the synthesis and characterization are provided in Supporting Information. Herein we mainly discuss the results of the Cis I (Mn = 8.6 kg/mol, PDI = 1.17), Trans I (Mn = 10.5 kg/mol, PDI = 1.13), Cis IV (Mn = 53.7 kg/mol, PDI = 1.49), and Trans IV (Mn = 49.45 kg/mol, PDI = 1.37) polymers. We

e continue to seek clear connections between the structure of polymer chains and their dynamics. Researchers have pursued a systematic approach to unraveling this connection by considering specifically poly(n-alkyl methacrylates) with side chains of various lengths.1−4 The latter exhibit two glass temperatures; a “conventional” high Tg and a polyethylene-like Tg at lower temperatures.1,2 In this work we have considered in particular how the polymer dynamics of such molecules varies with the stereochemistry of the side chain in the case that the stereochemistry is carefully controlled to yield two well-defined polymers, one with cis stereochemistry in the side group and one with trans stereochemistry in the side group. We demonstrate that controlled double-bond stereochemistry in the side group is the key ingredient for the multiple local segmental dynamics. It is known that polymers with double bonds in the repeat unit give rise to different cis−trans isomers with properties that depend strongly on the exact configuration of the double bond.5 Controllable double-bond stereochemistry influences the ability of polymers to crystallize and, hence, the mechanical properties.6−10 The well-known example is natural rubber and gutta percha. The superior elastomeric properties of natural rubber (rubber elasticity) reflect the stereoregular orientation and the concomitant inability of the backbone to crystallize. In addition, a higher glass temperature (Tg) was reported for poly(cis-isoprene) as compared to the amorphous phase in poly(trans-isoprene).7 Double-bond stereoregularity when incorporated in a side group could result to new polymers with controlled crystallinity and elasticity through the packing of side groups. A well-known example is polymers bearing azobenzene as a side group (azopolymers).11 However, cis© XXXX American Chemical Society

Received: October 9, 2017 Accepted: December 11, 2017

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DOI: 10.1021/acsmacrolett.7b00800 ACS Macro Lett. 2018, 7, 11−15

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ACS Macro Letters

initial structures were subjected to tight optimization criteria (RMS force criterion set to 1 × 10−5) under the DFTB3LYP15,16 level of theory and cc-pVTZ basis set.17 For all computations Gaussian 09 software package was employed.18 The DFT results revealed that the energetically more stable monomer configuration of the chain with trans side group stereochemistry (with relative energy of 2.4 kcal/mol) is one in which the entire side group is in an extended all-trans configuration, as shown in Figure 1. For the polymer with side group containing the cis double bond the most stable configuration (4.9 kcal/mol) contains a twisted side group at the position of the double bond. Depending on the dihedral angle d1, the dipole moment has values in the range from 0.2 to 2.0 ∼ debye. Both results from XRD and DFT calculations suggest a poorer packing ability for polymers with the cis configuration in their side group as a result of the angle formed by parts of the side group unfolded to the left and to the right of the double bond. Next we explore the influence of this structural feature on the segmental dynamics. Figure 2 depicts dielectric loss curves for the Trans I and Cis I polymers at different temperatures. The dielectric loss curves for Trans I depicts three processes (α1, α2, and β) corresponding to the dual segmental processes (α1, α2) and to a faster (β) process whose assignment will become clearer below. In contrast, Cis I has also three processes all assigned to the “segmental” dynamics. In addition, Trans I undergoes crystallization (at T = 258 K) via the side group promoted by the all-trans conformation. Details on the fitting procedure are provided in Supporting Information. The corresponding relaxation times are discussed with respect to Figure 3. Trans I displays two processes with a

employed temperature-modulated differential scanning calorimetry (TM-DSC), X-ray diffraction (XRD), and dielectric spectroscopy (DS) to study the structures and dynamics, respectively. XRD measurements were made with a D8 Advance diffractometer (Bruker AXS GmbH, University of Ioannina). Details on TM-DSC and DS are provided in Supporting Information. Figure 1 shows the X-ray patterns of four polymers with cis and trans configurations at ambient temperature. The WAXS

Figure 1. (Left) WAXS patterns of Trans I, Trans IV, Cis I, and Cis IV. The blue and red arrows indicate the position of low van der Waals peak and of the van der Waals peak, respectively. (Right) Chemical structures of Cis I and Trans I, as indicated, and respective optimized monomer configurations.

patterns show two main peaks. The peak at high q, with an equivalent Bragg spacing of about 4−5 Å, corresponds to the van der Waals (vdW) contacts of atoms and is known as the vdW peak. The peak at low q, which is usually referred to as the low van der Waals (LvdW) peak, reflect mainly the average distance between adjacent backbones.14,3,4 In both cases the results from the X-ray study reveal nanophase separated domains of backbones and alkyl side groups. However, polymers with the cis configuration have a systematically longer backbone-to-backbone distance as compared to polymers with the trans configuration. For example, the equivalent Bragg spacing is 3.03 nm in Trans I and 3.36 nm in Cis I. Similarly, the equivalent Bragg spacing is 3.12 nm in Trans IV and 3.34 nm in Cis IV. To explore the effect of stereoregularity on the packing and to further obtain the dipole moment for each configuration, DFT calculations were employed at the monomer level. Four

( ), where τ

VFT T-dependence, τmax = τ0 exp

B T − T0

0

is the

relaxation time in the limit of very high temperatures, B is the activation parameter, and T0 is the “ideal” glass temperature. The parameters for the α1 and α2 processes assume respective values, 10−12 s, 2560 ± 80 K, and 161 ± 1 K and 10−12 s, 2880 ± 25 K, and 136 ± 1 K. The faster β-process conforms to an

( RTE ), with an activa-

Arrhenius T-dependence, τmax = τ0 exp

tion energy of 36 ± 4 kJ/mol. The reversing heat capacity, shown in Figure 3c, depicts a very broad Tg that more closely reflects the freezing of α2 in DS. This situation is very different in the polymers with the cis configuration. Figure 3b depicts three processes with a VFT T-dependence with respective parameters: 10−12 s, 3100 ± 100 K, 134 ± 5 K, and 10−12 s, 2830 ± 50 K, 132 ± 2 K, and 10−12 s, 1640 ± 20 K, 137 ± 1 K,

Figure 2. Dielectric loss curves as a function of frequency for (a) Trans I (temperatures in the range from 258 to 328 K) and (b) Cis I (temperatures in the range from 208 to 298 K). The solid lines represent fits to a summation of three Havriliak−Negami functions in addition to the dc-conductivity contribution at lower frequencies. 12

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Figure 3. Activation plot of the different processes in (a) Trans I and (b) Cis I. Symbols in yellow are from TMDSC. The solid and dashed lines represent fits to the VFT and Arrhenius equation, respectively. The vertical line in (a) indicates the melting temperature of Trans I. Reversing heat capacity for Trans I (c) and Cis I (d) at different periods of modulation as indicated. Vertical arrows indicate respective Tgs.

dependence. On the other hand, the freezing of the α3 process in the cis polymers has weaker molecular weight dependence. These DS results are supported by rheology measurements (Supporting Information, Figures S19 and S20). The temperature dependence of the segmental relaxation times in the cis- and trans-polymers are also at variance from most studied homopolymers. The fragility parameter or the steepness index, m = ϑ log τ/ϑ(Tg/T)|T=Tg, can readily be calculated as, BTg/2.303(Tg − T0)2, and amounts to 33 (35) and 42 (36) for the Cis I and Trans I, respectively, for process α1 (α2). For α3 in the Cis I case, the steepness index is somewhat higher (52). These values, among the lowest reported for homopolymers,20,21 are consistent with the expectation born from poly(n-alkyl methacrylates), where the fragility (as well as Tg) decrease with an increase in the length of the flexible alkyl side group.4 The even lower values found here reflect, in addition, the influence of side group doublebond stereochemistry. Additional insight on the origin of the α1, α2 processes can be obtained by pressure-dependent studies (Trans I, Figure 5).22 Pressure slows down the segmental dynamics (Figure 5). The latter are described by the pressure equivalent to the VFT

for the α1, α2, and α3 processes, respectively. The reversing heat capacity trace shows two clear steps: one at lower temperatures that couples to the dielectric α3 process and a broader one at higher temperatures that couples to both α1 and α2 processes. The corresponding cooperative rearranging regions have correlation lengths of 1.5 and 1.1 nm for the lower and higher Tg, respectively (i.e., within the heterogeneity length scale of ∼3.3 nm; Figure S21).19 Similar results were obtained for Cis IV and Trans IV polymers (Figure S14). The results on Tg obtained from DS (defined in DS at τ = 102 s) and DSC are summarized in Figure 4 as a function of molecular weight. Evidently, the freezing of the segmental dynamics corresponding to α1, α2 processes are higher by 15 and 10 K, respectively, in the trans as compared to the cis polymers. This situation here is very different from polymers bearing stereoregularity with double bonds at the backbone.7 Furthermore, both α1, α2 processes display molecular weight

( ), where τ

equation23 as, τmax = τα exp

DPP P0 − P

α

is the segmental

relaxation time at atmospheric pressure at a given temperature, DP is a dimensionless parameter, and P0 is the pressure corresponding to the “ideal” glass. In addition, the slope at each pressure defines the apparent activation volume, defined as ΔV # = 2.303RT

(

∂ log τ , ∂P T

)

which relates to the volume of the

underlying dynamic processes.24,22 Strikingly, this quantity for both processes is of the order of the volume of the PMMA repeat unit, which is substantially smaller than the repeat unit volume of the polymer under investigation here. As the apparent activation volume depends also on pressure D P (ΔV # = RT (P −P P0 )2 )) this quantity can be obtained for both

Figure 4. Tg as a function of molecular weight obtained from DSC (polygons) and DS (squares, circles and triangles correspond to the respective freezing of the α1, α2, and α3 processes). Dashed (transpolymers) and dotted lines (cis-polymers) are guides for the eye.

0

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Figure 5. (a) Dielectric loss curves of Trans I as a function of frequency under isothermal conditions (T = 298 K). Pressure increases in the direction of the arrow: P = 0.1 MPa (circles), P = 30 MPa (squares), P = 60 MPa (up triangles), P = 90 MPa (down triangles), P = 120 MPa (rhombi), P = 150 MPa (left triangles), P = 180 MPa (right triangles), P = 210 MPa (hexagons), P = 240 MPa (stars), P = 270 MPa (pentagons). (b) Pressure dependence of the α1 (open symbols) and α2 (filled symbols) processes at different temperatures: T = 308.0 K (circles), T = 298.0 K (squares), T = 286.5 K (up triangles), and T = 276.9 K (down triangles). The solid and dashed lines are fits to the pressure equivalent to the VFT equation for the α1 and α2 processes, respectively, with Dp = 24.76. (c) Apparent activation volume at 0.1 MPa as a function of temperature. The dashed and dotted lines represent volumes corresponding to PMMA repeat unit and side group, respectively. (d) Pressure dependence of the glass and crystallization temperature of Trans I. The solid and dashed lines represent the Tg(P) and Tc(P) fits to glass temperature and Clausius−Clapeyron equations, respectively.

record of three VFT processes in a single component system. They supplant other poly(n-alkyl methacrylates) with long alkyl side groups. The key structural feature behind the variable dynamics here is double bond stereochemistry and, in particular, the inability of side groups bearing the cis double bond to pack efficiently. In summary, the synthesis of new polymers with trans- and cis-configurations as side groups enabled us to trace the richest dynamic behavior for polymer melts reported so far. This “dynamics engineering” approach was facilitated by the particular molecular design (double-bond stereochemistry) assisted by dipoles used as markers that sample both side group and backbone dynamics. These systems exhibit features that are not common to homopolymers. These include the presence of three segmental processes, the very low fragility index and the lowest pressure coefficient of Tg ever reported for polymers. For the cis-polymer, these unique features reflect on the inability of side groups to pack efficiently.

processes and is plotted in Figure S17, Supporting Information. Figure 5d depicts the pressure dependence of the corresponding Tg as well as of the crystallization temperature (Tc). The latter, as a first order transition, conforms to the Clausius− Clapeyron equation, dP/dT = ΔH/TΔV, where ΔH is the change in enthalpy at the transition and ΔV the corresponding volume change. The former can be described as

(

ν

Tg(P) = Tg(0) 1 + μ P

1/ ν

)

, with T g (0) being the glass

temperature at atmospheric pressure and μ and ν fitting parameters with values 241 K, 2100 MPa, 1.7 and 224.9 K, 2160 MPa, 5.7 for the α1 and α2 processes, respectively. In addition, the pressure coefficient of Tg, dTg/dP|P→0, is 115 and 104 K/GPa for the α1, α2 processes, respectively. These are the lowest pressure coefficients of Tg reported for homopolymers.25 The higher apparent activation volume for α1, results in a better separation of the two processes at elevated pressures. Based on the combined results from the T- and P-dependent DS investigation the three VFT processes in the cis-polymers are assigned as follows: The faster process (α3) bearing a high effective dielectric strength (TΔε ∼ 150 K) and a well-resolved Tg (ΔCp ∼ 0.4 J/gK) is associated with the side-group relaxation involving the ester group dipole in proximity to the double bond. The two slower processes (α1, α2), with respective dielectric strengths of 175 and 65 K, reflect the backbone dynamics as sampled by the two ester dipoles. The proximity of the dipoles results to the observed coupled dynamics. All processes are affected by packing. For example, α3, exist in the cis-polymers because of the loose packing in side groups resulting from the large angle of different parts of the side group relative to the cis double bond. In addition, α1 and α2, are faster in the cis-polymers (lower Tgs) as compared to trans-polymers, for the same reason. The cis-polymers hold the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00800.

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Details on TM-DSC, DS experiments, rheology, and DFT calculations (PDF).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: gfl[email protected]. *E-mail: [email protected]. 14

DOI: 10.1021/acsmacrolett.7b00800 ACS Macro Lett. 2018, 7, 11−15

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Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision E.01.; Gaussian, Inc.: Wallingford, CT, 2009. (19) Donth, E. The Glass Transition; Springer-Verlag: Berlin, Heidelberg, 2001. (20) Ngai, K. L.; Roland, C. M. Chemical Structure and Intermolecular Cooperativity: Dielectric Relaxation Results. Macromolecules 1993, 26 (25), 6824−6830. (21) Kunal, K.; Robertson, C. G.; Pawlus, S.; Hahn, S. F.; Sokolov, A. P. Role of Chemical Structure in Fragility of Polymers: A Qualitative Picture. Macromolecules 2008, 41 (19), 7232−7238. (22) Floudas, G.; Paluch, M.; Grzybowski, A.; Ngai, K. L. Molecular Dynamics of Glass-Forming Systems; Springer: Berlin, 2011. (23) Paluch, M.; Patkowski, A.; Fischer, E. W. Temperature and Pressure Scaling of the a Relaxation Process in Fragile Glass Formers: A Dynamic Light Scattering Study. Phys. Rev. Lett. 2000, 85, 2140− 2143. (24) Floudas, G.; Gravalides, C.; Reisinger, T.; Wegner, G. Effect of pressure on the segmental and chain dynamics of polyisoprene. Molecular weight dependence. J. Chem. Phys. 1999, 111, 9847−9852. (25) Panagos, P.; Floudas, G. Dynamics of poly(propyl methacrylate) as a function of temperature and pressure. J. Non-Cryst. Solids 2015, 407, 184−189.

George Floudas: 0000-0003-4629-3817 Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acsmacrolett.7b00800 ACS Macro Lett. 2018, 7, 11−15