A New Approach to Dynamic Vulcanization: Use of Functional

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A New Approach to Dynamic Vulcanization: Use of Functional Nitroxyls to Control Reaction Dynamics and Outcomes Michael Bodley, and J Scott Parent Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02681 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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Industrial & Engineering Chemistry Research

A New Approach to Dynamic Vulcanization: Use of Functional Nitroxyls to Control Reaction Dynamics and Outcomes Michael W. Bodley and J. Scott Parent* Department of Chemical Engineering Queen’s University Kingston, Ontario, CANADA, K7L 3N6

Abstract A new method of exerting control over the dynamics and outcomes of radical-mediated polyolefin modifications is adapted for the dynamic vulcanization (DV) of polypropylene + poly(ethylene-co-octene) (PP+EOC) blends. Whereas the conventional peroxide-initiated DV of these blends produce extreme initial rates of EOC crosslinking and extensive PP matrix degradation, formulations containing nitroxyls that bear polymerizable functionality do not suffer from these limitations. Acryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl (AOTEMPO), used alone or in combination with a triacrylate monomer, is used to delay the onset of polymer modification, raise the crosslink density of dispersed elastomer phase, and mitigate changes to the molecular weight of the thermoplastic matrix. Brief demonstrations of the influence of AOTEMPO on dicumyl peroxide (DCP) modifications of each blend component is followed by careful studies of PP+EOC dynamic vulcanization. Dispersed phase size distributions and melt-state oscillatory rheometry reveal the effect of AOTEMPO on DV product architecture and morphology.

Graphical Abstract P O N 9

-1 -1

kcomb=10 M s

O.

fast

N

+P

O

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O O

O kadd=105 M-1s-1

N

O slow

O

. O

P

* Author for correspondence Email: [email protected]

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Introduction Dynamic vulcanization (DV) of immiscible polymer blends has emerged as an important technology for transforming commodity materials into value-added products. It is used primarily to prepare thermoplastic vulcanizates (TPVs) comprised of finely dispersed, crosslinked elastomer particles distributed throughout a continuous thermoplastic matrix.1,2 Their compositions range from impact-modified plastics that contain a small amount of elastomer,3 to elastomer-rich, soft TPVs whose mechanical properties approximate those of cured rubber articles.4,5 Since thermoplastic makes up the continuous phase, DV products are thermoformable and can be processed by conventional, high-throughput equipment. Moreover, the crosslinked elastomer phase is not susceptible to coalescence, thereby eliminating concerns over blend morphology coarsening during melt processing.6

DV generates these dispersed-phase structures by solvent-free, reactive polymer blending.7 Typically, the elastomer is mixed with curatives and charged with the thermoplastic to a conventional polymer compounding device.8 Mixing is done above the melting point of the thermoplastic, with blend morphology evolving concurrently with chemical modification of one or both polymer components. Unlike standard polymer blending processes that can achieve a steady-state morphology in a very short timeframe, 9 DV involves continuous interaction of process chemistry, melt-state rheology, and blend morphology.10 This gives rise to enormous complexity, and the commercial development of DV technology has outpaced studies of its fundamental principles.

Significant knowledge has been gained regarding TPV structure-property relationships11 and the evolution of blend morphology during reactive processing.12 However, the limitations of conventional polymer crosslinking chemistry remain a significant constraint. DV temperatures are more extreme than those encountered in standard elastomer cures, necessitating the use of formulations that produce robust carbon-carbon crosslinks as opposed to reversion-prone sulfidic networks. Demands on reaction kinetics are also severe. While short reaction times are the norm, in many cases the elastomer phase reaches the gel point before the thermoplastic has fully melted. Excessive crosslinking in the early stages of a curing process is known as “scorch”, and its implications for blend morphology are a key concern in DV process development.13

An ideal DV formulation leaves the thermoplastic phase unaffected while crosslinking the elastomer phase with tunable dynamics and yields.14 Acid-activated resol chemistry satisfies the selectivity criterion for blends of polypropylene (PP) with unsaturated elastomers, since its cure mechanism involves electrophilic attack of 2 ACS Paragon Plus Environment

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cationic intermediates on the elastomer’s olefin functionality.15 However, these products suffer from discoloration and water absorption problems, and contain residues of tin and zinc cure activating agents.16 Peroxide-based formulations do not suffer from these issues, and are capable of crosslinking saturated polyolefins such as poly(ethylene-co-octene) (EOC) to give a thermally stable C-C bond network. Unfortunately, migration of the initiator into the PP phase results in radical-mediated chain scission. Therefore, there remains a need for new radical chemistry that is capable of crosslinking an ethylene-rich elastomer while having little or no effect on the molecular weight of PP.

One means of improving the scorch safety of a peroxide formulation is to add an antioxidant to trap radical intermediates in the early stage of the curing process.17,13 We have recently developed functional nitroxyls such as acryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl (AOTEMPO) to control the dynamics and yields of polyolefin modifications (Scheme 1).18 Our first application of this chemistry set out to delay the onset of polyethylene crosslinking without compromising the network density of the resulting thermoset.19 We then extended this approach to scission-prone polymers such as PP that degrade under the action of peroxide alone, but can be crosslinked extensively when formulated with small amounts of functional nitroxyl.20 In this work, we explore the potential of AOTEMPO to overcome challenges associated with peroxide-initiated DV of PP+EOC blends. Our objective was to assess the ability of AOTEMPO to influence the progress of DV reactions as well as product morphology and melt-state rheology.

Scheme 1: Reagent structures O O

N O O

O

O O

O

O

AOTEMPO

TMPTA

This report begins with studies of individual polymer formulations that establish the sensitivity of each material to peroxide, AOTEMPO and coagent. This is followed by DV experiments on PP+EOC blends of intermediate composition. The intent is to demonstrate the potential of AOTEMPO chemistry to advance TPV technology, thereby fuelling further application development research.

Experimental



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Materials. Poly(ethylene-co-octene) (EOC, Engage 8003, 7.6 mol% octane, density=0.885 g/cm3, MFR = 1.0 g/10min@190°C) was used as received from Dow Chemical. Polypropylene (PP, Pro-fax 6523, Mw= 340 kg/mol, density = 0.900 g/cm3, MFR = 4.0 g/10 min@230°C) was used as received from Lyondell Basell. Dicumyl peroxide (DCP, 99%), trimethylolpropane triacrylate (TMPTA, 98%), and pentaerythritol tetrakis(3,5-di-tert-butyl-4hydroxyhydrocinnamate) (Irganox1010, 98%) were used as received from Sigma-Aldrich. AOTEMPO was prepared as previously described.

TPV Synthesis. PP + EOC formulations (80:20, 70:30, 60:40, 50:50, wt:wt) containing DCP (0.00 or 9.25 µmol/g·polymer), TMPTA (0.00 or 33.75 µmol/g·polymer), and AOTEMPO (0.00 or 12 µmol/g·polymer) were compounded with a Haake Polylab R600 internal batch mixer using a two-stage procedure. Masterbatches comprised of EOC (40 g) and the required curatives were mixed at 90 °C and 60 rpm for 5 min and cut into pellets. The required amount of PP was mixed at 180 °C, 60 rpm for 4 min before adding the required amount of masterbatch. Mixing was continued for a 5 min at 60 rpm before adding Irganox 1010 (0.5 wt%, 4.25 µmol/g) and quenching a portion of the product in ice-water to preserve sample morphology. For example, DV of a 70:30 wt:wt blend involved preparing a masterbatch of EOC (40 g) containing DCP (0.333 g, 1232 µmol) TMPTA (1.3 g, 4500 µmol) and AOTEMPO (0.435 g, 1924 µmol). PP (28 g) was charged to the preheated mixer and combined with an aliquot of the elastomer masterbatch that contained EOC (12 g), DCP (0.1 g, 370 µmol), TMPTA (0.4 g, 1350 µmol), and AOTEMPO (0.131 g, 577 µmol).

Gel Content Analysis. Gel contents were measured according to ASTM D 2765-01. A weighed sample was enclosed in 120 mesh stainless steel cloth, and extracted with boiling xylenes using BHT to mitigate polymer oxidation. Extraction was continued for 4 hr, after which the sample was dried to constant weight, and the gel content reported as a percent of mass retained from the original sample. Analysis of TPVs produced widely variable gel contents due to inevitable loss of EOC particles from the extraction cloth. However, these analyses confirmed crosslinking of the EOC phase.

Rheological Analysis. Individual PP or EOC formulations were reacted in the melt-sealed cavity of a controlledstrain rheometer (Advanced Polymer Analyzer 2000, Alpha Technologies) equipped with biconical plates operating at 180 °C, 1 Hz and 3° arc. Samples were prepared by grinding the polymer (5 g), coating it with an acetone solution of the desired reagents, and allowing the acetone to evaporate before charging the mixture to the rheometer. TPV samples that were prepared in the Haake Polylab were analysed with a controlled strain


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rheometer (Anton Parr MCR301) equipped with 25 mm parallel plates, using a 1 mm gap at 170 °C, and 3% strain. Stress sweeps were performed to ensure that data acquired was within the linear viscoelastic region.

TPV Morphology. Samples were immersed in liquid nitrogen for 5 min before fracturing. Surfaces of interest were etched for 2.5 hr using heptane at 80 °C. Unreacted blends were immediately dried under vacuum at 60 °C for 10 hr. TPVs were not etched by this simple extraction process. Rather, the crosslinked EOC phase in samples was swelled with heptane as described above, then sonicated using a Misonix XL2000 microson ultrasonic probe for three 1 min intervals at a power output of 22 watts. This treatment dislodged particles from the surface, facilitating SEM imaging. The resulting etched samples were gold-coated and imaged using a Hitachi S-2300 scanning electron microscope. Images were analyzed using ImageJ, imaging analysis software, to estimate the mean diameter of the dispersed elastomer phase, as well as the polydispersity index (PDI) as defined below.21 =

∑ ∑

∑

= ∑

=

TPV Mechanical Properties. Solid-state flexural tests were performed according to ASTM D790 at a speed of 1.3 mm/min on rectangular bars (dimensions 125 x 13.1 x 3.05 mm) prepared by compression molding at 180°C. Flexural modulus data are provided as Supplemental Information. Notched Izod impact tests were carried out according to ASTM D256 using a Instron BLI impact tester at room temperature on specimens (dimensions 62.5 x 12.5 x 3.1 mm) prepared by compression molding at 180°C. At the EOC loadings used in this work, brittle fracture was only observed for unfilled PP.

Results and Discussion Modification of Individual Blend Components The outcome of a conventional peroxide modification is dictated by the intrinsic reactivity of alkyl macroradical intermediates. In the case of ethylene-rich polyolefins, macroradical combination is the dominant molecular weight-altering reaction, resulting in polymer crosslinking through C-C bond formation. In the case of propylenerich polyolefins, β-scission of tertiary macroradicals overwhelms macroradical termination by combination, and peroxide modification of PP homopolymers generally results in large-scale degradation. Consider the rheology data plotted in Figure 1 for the peroxide-initiated modifications of EOC and PP. Storage modulus (G’) measurements, recorded at a fixed temperature, frequency, and shear strain amplitude, are a standard means of monitoring the progress of reactions that affect polymer architecture.22 An EOC formulation comprised of DCP alone crosslinked quickly upon heating to the reaction temperature to produce a net gain in G’ of 150 kPa



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and a gel content of 62% (Figure1a). In contrast, the DCP-only formulation applied of PP resulted in substantial polymer degradation, with a net loss in G’ of 12 kPa over the same period (Figure1b).

Further challenges arise from the inherent dynamics of peroxide-initiated polyolefin modifications. These processes generate radicals through initiator thermolysis, a first-order reaction with half-life of 0.86 min for DCP at 180°C,23 while radical termination by combination and/or disproportionation occurs at the diffusion limit of reaction velocilty.24 With peroxide decomposition as the rate determining step, overall process dynamics are inherently first-order, with reactions proceeding fastest in the initial stages, when the peroxide concentration is highest, and slowing exponentially as the initiator is consumed. These chemical kinetics are apparent in the DCPonly EOC crosslinking and PP degradation data plotted in Figure 1.

Formulations containing the trifunctional monomer trimethylolpropane triacrylate (TMPTA) were of preliminary interest, given their frequent appearance in DV literature. These additives promote polyolefin crosslinking by graft addition and oligomerization reactions of their multiple C=C bonds. When applied to our ethylene-rich elastomer, TMPTA produced the expected response, improving the EOC crosslink yield while greatly accelerating the initial cure rate (Figure 1a). The effect of TMPTA on our PP homopolymer was consistent with recent reports of long chain branching.25 A thorough review of the architecture of coagent-modified PP is beyond our scope, and the reader is referred to specific studies devoted to the subject.26 Briefly, coagent grafting to PP generates bimodal molecular weight and branching distributions, with most chains having a linear structure and reduced molecular weight, and a minority chain population having a hyperbranched structure with molecular weights extending to the gel point. Although the hyperbranched fraction contributes to melt elasticity at low shear rates, the degraded linear matrix dominates rheological properties at the high shear rates where TPVs are processed. The storage modulus data plotted in Figure 1b for DCP+TMPTA shows clear evidence of this degradation, as the final G’ produced by DCP+TMPTA was only marginally higher than that produced by DCP alone. The inability of a DCP+TMPTA formulation to provide the requisite control over EOC cure dynamics and PP molecular weight led us to eliminate it from further investigation.


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Figure 1- Evolution of G’ and dG’/dt for peroxide-initiated polyolefin reactions at 180 oC, 1Hz, 3° arc (a. EOC; b. PP; [DCP]= 9.25 µmol/g; TR=0.65; [TMPTA]= 33.75 µmol/g)

The ability of nitroxyls such as AOTEMPO to provide a measure of control over the dynamics and yields of polyolefin modifications has been demonstrated previously.27 These reactions progress through a sequence of three phases; induction, C=C oligomerization, and uncontrolled polymer modification. During the induction phase, trapping of carbon-centered radicals by combination with nitroxyl quenches macroradical intermediates. Since the polymer’s molecular weight distribution remains constant until all nitroxyl is consumed, the induction time is a simple function of the peroxide half-life and the amount of AOTEMPO charged to the formulation.28 The latter is generally expressed as the trapping ratio, TR = [AOTEMPO]/(2*[DCP]), which represents the fraction of initiator-derived radicals that will be quenched by the additive. In general, the induction time is not a function of polyolefin structure, as demonstrated by the common 1.8 min induction time for the DCP+AOTEMPO modification of both EOC and PP (Figure1).

The second phase of an AOTEMPO process builds a covalent network by oligomerization of polymer-bound acrylate functionality. In the case of EOC, the cure rate is accelerated relative to peroxide alone, since C-C bond formation occurs by functional group activation as well as macroradical combination (Figure1a). In the case of PP, crosslinking competes with macroradical scission, the balance of which dictates the net change in storage 7 ACS Paragon Plus Environment


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modulus. This balance is affected by polymerizable group structure, trapping ratio, and initial PP molecular weight. Although the DCP+AOTEMPO data summarized in Figure1b show a small extent of degradation, formulations can be designed to meet a range of product architectures, from degraded to slightly branched to thermoset.

Phase three of the process, uncontrolled modification, is marked by the complete conversion of acrylate functionality, at which point residual peroxide further crosslinks EOC, and further degrades PP. Optimization of the trapping ratio can minimize this stage by supplying only as much initiator as is required to convert polymerbound C=C functionality. Given the current state of knowledge, this optimal trapping ratio can only be determined by trial and error experimentation.

While a DCP+TMPTA formulation cannot meet our DV performance objectives, synergy between AOTEMPO and TMPTA can be used to positive effect. When used in isolation, acrylic coagents suffer from low polymer graft yields, due to a kinetic preference for C=C oligomerization. When used in combination with AOTEMPO, TMPTA only needs to copolymerize with polymer-bound acrylate groups during the oligomerization phase of the cure to expand the crosslink network. As a result, DCP+AOTEMPO+TMPTA raised the crosslink density of the EOC system, and shifted the PP modification outcome from degradation to slightly crosslinking (Figure 1). Since these yield enhancements were gained without compromising the nitroxyl induction delay, this formulation has considerable potential to advance DV technology.

Dynamic Vulcanization Studies The challenge in developing a controlled DV process is to find a peroxide concentration, coagent loading, and nitroxyl trapping ratio that provides the requisite induction time, EOC crosslink density and PP matrix viscosity. The initiator and AOTEMPO concentrations used to generate Figure1 ([DCP]=9.25 µmol/g, TR=0.65) generated an induction delay of 1.8 min at 180°C, as well as favourable polyolefin modification outcomes. Therefore, DV studies centred on these reagent loadings, with four cure formulations used to assess the efficacy of our functional nitroxyl chemistry. An unreactive blend generated baseline data on the morphology and rheological properties inherent to the starting materials, whereas the DCP-only formulation benchmarked the performance of a conventional peroxide-mediated synthesis. DCP+AOTEMPO data revealed the influence of the functional nitroxyl in isolation, while reactions conducted with DCP+AOTEMPO+TMPTA demonstrated its influence in combination with a synergistic coagent. Each of these formulations was applied to four PP:EOC blend ratios


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(80:20, 70:30, 60:40, and 50:50 wt:wt) to assess the performance of AOTEMPO over a range of TPV compositions.

All DV reactions were performed in an internal mixer operating at 60 rpm and a set point temperature of 180°C, with all polymers and reagents added a a single charge at time zero. Note that the device cannot provide isothermal conditions, since the EOC masterbatch absorbs heat from the PP melt when charged to the mixer, and mechanical shear generates heat throughout the process. This non-isothermal character, as well as the continuous shear field imposed by the instrument, differentiates these DV experiments from the single-polymer rheology studies discussed above. Whereas the storage modulus data illustrated in Figure 1 are sensitive only to polymer structure, the applied torque data plotted in Figure 2 for are sensitive to the rheological properties of each phase as well as the blend morphology.29

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Figure 2 – Melt temperature and applied torque of dynamically vulcanized 70:30 PP:EOC blends ([DCP]= 9.25 µmol/g; TR=0.65; [TMPTA]= 33.75 µmol/g). 9 ACS Paragon Plus Environment


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Processing a 70:30 mixture without any curatives produced a simple torque profile (Figure 2). An initial spike was observed as EOC was added to the device, and the torque plateaued within one minute of melt mixing, declining only slightly in response to increasing melt temperature. Based on reports of morphology development in other unreactive blends,30,31 this stable plateau reflects the intrinsic morphology of the starting materials under these melt-processing conditions.9,32 At this point, droplet breakup and coalescence have established a dynamic equilibrium that can respond to changes in material properties and/or mixing intensity, but remains stationary since no changes are imposed on the system.

The response observed for the peroxide-only formulation is consistent with large scale degradation of the thermoplastic matrix. Although the EOC phase is cured by DCP-alone, scission of the continuous PP phase dominates the TPV melt viscosity at the shear rates imposed by the mixer, and torque values recorded after 4 min of processing time were 50% lower than those of the unreactive blend. This loss in melt viscosity is a key limitation of conventional peroxide modification technology, as the product lacks the melt strength required by a range of polymer processing operations.

The DCP+AOTEMPO and DCP+AOTEMPO+TMPTA data are the cornerstone of this report, as they demonstrate the degree to which the dynamics and yields of a dynamic vulcanization can be controlled using a functional nitroxyl additive. The initial torque profiles generated by these formulations were indistinguishable from that of the unreactive blend, since macroradical trapping by nitroxyl stabilized the melt viscosity of each phase while introducing pendant acrylate functionality. This is a distinguishing feature of a nitroxyl-mediated strategy,17 as it allows the starting materials to establish their inherent blend structure before the elastomer is rendered thermoset, unlike conventional formulations that crosslink rapidly from the beginning of polymer processing.30,33 An abrupt increase in applied torque marked the transition from phase 1 of the cure to phase 2, as polymerbound acrylate oligomerization crosslinked the EOC phase extensively, while PP underwent simultaneous crosslinking and chain cleavage to the different extents, depending upont TMPTA availability. Reversion is evident in both AOTEMPO formulations, due in part to increases in melt temperature, but likely augmented by changes in PP matrix viscosity and blend morpohology. More precise information regarding the properties of the final products is provided below.


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DV Product Morphology Scanning electron microscopy (SEM) analysis of our 70:30 samples provided insight into TPV morphology (Figure 3). All materials were comprised of a continuous PP matrix, and a dispersed EOC phase, as expected given the predominant PP volume fraction. The dispersed phase morphology developed in the unreacted blend was quite uniform, with a mean droplet diameter of 1.5 µm and a PDI of 1.35. This degree of EOC droplet dispersion is consistent with expectations based on the relative viscosities of the blend components. Crosslinked EOC particles found in the TPV produced by peroxide alone were more irregularly shaped and slightly larger, with a mean diameter of 1.73 µm and a PDI of 1.22. These are values are consistent with those reported similar TPV systems.4

Figure 3 - SEM images of etched 70:30 blend surfaces: (a) unreacted, (b) peroxide, (c) peroxide + AOTEMPO, (d) peroxide + AOTEMPO + TMPTA.

The DV products prepared from AOTEMPO formulations had significantly better EOC phase dispersions than the unreacted blend, with AOTEMPO alone producing EOC particles with diameters of 1.2 µm and a PDI of 1.18, and 11 ACS Paragon Plus Environment


AOTEMPO+TMPTA generating 0.6 µm with a PDI of 1.28. This result is particularly interesting in light of the applied torque data recorded during sample preparation (Figure 2). Since the induction phase of an AOTEMPOmediated cure produces no change in polyolefin properties, the blend morphology established during this period is that of the unreacted starting materials. Crosslinking of the EOC phase during the oligomerization stage of the cure raises the dispersed phase viscosity markedly, while having a more subdued effect on the PP matrix. Despite the speed of acrylate oligomerization, it does not “lock in” the morphology of the unreacted blend. Rather, blend morphology continues to evolve during the oligomerization and uncontrolled modification phases of the cure process.

3.5

EOC Phase Diameter (um)

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1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 80/20

70/30

60/40

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PP:EOC Blend Ratio (wt:wt)

Figure 4 - Mean diameter and polydispersity index of the dispersed EOC phase in DV blends


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The performance of AOTEMPO on the 70:30 blend carried over to other polyolefin ratios, with the nitroxylmediated formulations providing better EOC particle dispersions than unreactive blends and peroxide-only TPVs (Figure 4). Although particle diameters increased with the EOC weight fraction, the 50:50 materials retained a dispersed phase morphology, avoiding potential co-continuity. Images for 80:20, 60:40, and 50:50 blends can be found in the Supplemental Information. Careful studies of the erosion of gelled elastomer domains in a thermoplastic matrix have highlighted the importance of stress break-up of rubber domains.34,35 In the present context, the preservation of PP melt viscosity by AOTEMPO formulations is expected to enhance breakup during the post-induction period, in sharp contrast to peroxide-only formulations that incur extensive matrix degradation.

DV Product Rheology Knowledge of melt-state rheological properties not only supports the development of polymer processing operations, it can provide insight into molecular weight distributions, chain architectures and blend morphologies. Given that comprehensive reports on the melt-state properties of PP-based TPVs are available,36 the scope of our study was limited to the effect of AOTEMPO-mediation on DV products. We began with a brief analysis of how AOTEMPO affects the properties of PP alone before progressing to studies of corresponding PP+EOC blends. Further details regarding the structure and rheological properties of AOTEMPO-modified EOC and PP are described elsewhere.19,20

Figure 5a provides complex viscosity (η*), G’, and phase angle measurements for unmodified PP and its peroxide-modified derivatives. The molecular weight of our PP homopolymer is demonstrated by a high complex viscosity and pronounced shear thinning character. Its linear structure limited its chain entanglement potential, leading to relatively efficient stress relaxation at low frequency. This is evident in the approach of η* toward a Newtonian plateau, and the scaling of G’ with ω2, which is indicative of a terminal flow condition. As expected, peroxide-only degradation lowered complex viscosity and storage modulus dramatically, while elevating low-frequency phase angle measurements toward δ=90o. This purely viscous response to an oscillatory deformation contrasts with that of a purely elastic material, for which the stress and strain are in-phase (δ=0o). 37 Taken together, these rheology data are consistent with a low molecular weight, linear chain architecture.38



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70 60 50 40 30 20

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Frequency (1/s)

Figure 5 – η*, G’, and phase angle versus frequency a. PP; b. 70:30 PP:EOC TPVs (170°C)

The ability of an AOTEMPO formulation to mitigate changes in the melt viscosity of polypropylene depends on the extent to which a branched architecture can approximate the rheological properties of its linear PP parent material. This is because, by design, AOTEMPO preserves molecular weight by crosslinking of polymer-bound acrylate groups in attempt to compensate for concurrent backbone scission. Applying DCP+AOTEMPO to PP alone gave a product whose complex viscosity and storage modulus were intermediate between the starting polymer and peroxide-degraded material (Figure 5a). Moreover, this PP derivative exhibited the Newtonian plateau and low-frequency phase angle that is characteristic of a substantially linear polymer. In contrast, the product of a DCP+AOTEMPO+TMPTA formulation showed unmistakable evidence of a long chain branched architecture, with low-frequency properties responding strongly to the stress relaxation restrictions arising from 14 ACS Paragon Plus Environment

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enhanced chain entanglement.39 Optimization of peroxide and nitroxyl loadings can be used to tailor these melt flow properties to achieve different outcomes, but as we noted above, adjustments to a DV formulation must also consider potential changes to elastomer crosslinking dynamics and yields.

Rheological data acquired for the 70:30 PP:EOC formulations are plotted in Figure 5b. Complex viscosity, storage modulus and phase angle measurements for the unreacted blend were similar to those made on the PP starting material, with some evidence of low-frequency elasticity attributed to dispersed EOC droplets. In contrast, the peroxide-only TPV, while showing compelling indications of PP matrix degradation, demonstrated remarkable low-frequency elasticity. Given that radical-mediated PP degradation cannot introduce significant amounts of long chain branching, this diminished relaxation is attributed to the reinforcing effects of dispersed, cross-linked EOC particles. An early study of the steady shear rheology of conventional PP+EPDM TPVs characterized their behaviour as that of highly filled fluids, with the crosslinked phase serving as a particulate reinforcing agent.40 This rheological model has been adopted by several subsequent studies of TPV systems,41,8 and is consistent with the results of this work. Dispersed EOC particles eliminated the Newtonian plateau in the peroxide-only TPV, giving rise to the shear yielding behaviour frequently reported for this class of materials.42,43

These effects were most pronounced for the DCP+AOTEMPO+TMPTA formulation, as this formulation introduces long chain branching to the PP matrix along with a fine dispersion of crosslinked EOC particles. The complex viscosity and storage modulus of this TPV coincide well with the unreacted blend, especially at higher frequency, proving that AOTEMPO chemistry can mitigate viscosity losses during DV, in addition to providing a means of controlling the process dynamics. The rheological measurements plotted in Figure 6 expand the data set to other polyolefin ratios, demonstrating that higher EOC contents result in greater melt reinforcement.



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Figure 6 - Melt-state rheology data DCP+AOTEMPO+TMPTA applied to four PP:EOC blend ratios

Conclusions The utility of peroxide cure chemistry for the DV of polyolefin blends is limited by poor control over reaction dynamics and yields. Formulations containing a small amount of AOTEMPO delay the onset of polyolefin modification to allow the blend to achieve a steady-state morphology. Moreover, formulations can be designed to enhance the crosslink density of the dispersed elastomer phase while mitigating losses in thermoplastic melt viscosity. The products demonstrate superiour elastomer particle dispersion, and melt-stat rheological properties that can be manipulated using the reinforcement effects of crosslinked particles as well as long chain branching within the thermoplastic matrix. 16 ACS Paragon Plus Environment

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Supporting Information. SEM images of etched 80:20, 60:40, 50:50 PP:EOC blends and flexural modulus data.

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A New Approach to Dynamic Vulcanization: Use of Functional

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