Viscoelastic Behavior of Unentangled POSSâStyrene

Article pubs.acs.org/Macromolecules

Viscoelastic Behavior of Unentangled POSS−Styrene Nanocomposites and the Modification of Macromolecular Dynamics Angel Romo-Uribe* R&D, Advanced Science & Technology Division, Johnson & Johnson Vision, Jacksonville, Florida 32256, United States ABSTRACT: The viscoelastic properties of molten unentangled hybrid nanocomposites poly[(propylmethacryl-heptaisobutyl-POSS)-co-styrene], denoted POSS-sty, were investigated; POSS content varied up to 45 wt %. Unentangled polystyrene (PS) was also investigated. Differential scanning calorimetry (DSC) showed that the glass transition temperature, Tg, significantly decreased relative to neat PS, i.e., ΔTg < 0, and this effect was not associated with molecular weight. Master curves were constructed using the time−temperature superposition (TTS) principle. The shift factors obeyed Arrhenius-type relationship, and the calculated flow activation energy Ea first increased and then rapidly decreased as POSS content increased. The viscoelastic spectra of PS and the nanocomposites exhibited only the terminal regime (G′′ > G′), whereas PS exhibited the typical Rouse behavior; POSS-sty melts did not obey the scaling G′′ ∼ ω2. Strikingly, the mechanical damping tan δ exhibited minima at the longest relaxation time, not seen in melts of unentangled flexible polymers. This suggests (weak) interactions/ associations between the bulky POSS groups giving rise to dynamics retardation. The fractional free volume fg exhibited considerable increase, and the zero-shear viscosity η0 decreased an order of magnitude, relative to the neat polymer. Analysis of several POSS-based nanocomposites showed that ΔTg was positive or negative, in all cases produced viscosity reduction, and molecular weight was not a factor. As entanglements effects were removed in this styudy, the results suggest a modified polymer dynamics driven by interactions/associations of the bulky POSS macromers, thus opening opportunities for tunability of bulk properties in nanocomposites and the development of advanced functional materials.

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others, methacrylates,2,10,11 styrenes,3,4,8 epoxies,12−14 polyimides,15,16 urethanes,6,17,18 norbornenes,7 siloxanes,19 and polyesters.20 Recently, newly developed multifunctional POSS macromers, where 2, 4, or 8 corners are able to react, are opening up opportunities in bottom-up materials design. Hence, the influence of multifunctional POSS on epoxy networks,21 thermoplastic poly(propylene oxide),22 sulfonated polystyrene,23 low-dielectric polyimides,24 and hydrogels25 is being investigated. Applications of POSS polymers mono- and multifunctional are broadening and range from electronics,26−30 medical engineering,31−36 shape memory properties,37−41 to nonlinear optics.42 Several reviews have been published on POSS nanocomposites and its applications.1,9,43 A number of studies have revealed a strong dependence of the glass transition temperature on the level of copolymerization of POSS with such parent polymers as polystyrene, poly(methyl methacrylate), polynorbornene, polyamides, and poly(dimethylsiloxane). Because of the absence of polar units in POSS-mers, it is currently hypothesized that the mechanism by which the glass transition temperature is increased through POSS incorporation is by dynamics retardation over large

INTRODUCTION Polyhedral oligosilsesquioxane (POSS) macromer is represented by the formula P1R7Si8O12 with an inorganic silica-like core (Si8O12) surrounded by eight organic corner groups (P1R7). In early research and development efforts, the seven organic groups played the role of compatibilizers and/or solubilizers in a polymer matrix and were usually inert. Thus, the remaining corner was reactive and varied, thus enabling a broad range of nonreactive or reactive POSS macromers.1−3 Therefore, monofunctional POSS macromers provided the materials scientist with a novel and versatile reagent for introducing and manipulating many properties of established polymer systems. POSS is a nanosized molecule (with molecular weight ∼1000 g/mol) with an inorganic silica-like (SiO1.5) core surrounded by a shell of organic vertex groups. Thus, POSS can be viewed as a spherical-like nanoparticle, and it is about 5 times larger than most monomers. The bulkiness of POSS is considered responsible for modifications to polymer dynamics,4−8 giving rise to unusual physical properties and challenging the theoretical polymer physicists. The benefits of POSS macromer derivatives incorporated into polymeric materials comprise the increase in use temperature, corrosion resistance, surface hardening, and improved mechanical properties as well as reduction in flammability, heat evolution, and viscosity reduction useful for processing.9 These enhancements have been shown to apply to a wide range of thermoplastics and thermoset systems, among © XXXX American Chemical Society

Received: July 31, 2017 Revised: August 28, 2017

A

DOI: 10.1021/acs.macromol.7b01645 Macromolecules XXXX, XXX, XXX−XXX

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dynamics retardation in otherwise unentangled macromolecules.

length scales (polymer dimensions) of the polymer chain motion, either via intermolecular interactions or associations between POSS units or by the large inertia exhibited by a polymer segment containing the massive POSS-mers.4 Some of these studies comprise the thermal and viscoelastic behavior of POSS−styrene and POSS−norbornyl copolymers3,4,7 and POSS macromers dispersed in polymers.44 Other studies, however, have revealed strong reduction of glass transition temperature, for instance isobutyl-POSS-styrene copolymers, iBuPOSS.8 Moreover, rheological measurements revealed that the incorporation of iBuPOSS decreased the rubbery plateau modulus (G0N), suggesting a strong dilation effect of iBuPOSS on entanglement density. Moreover, the apparent flow activation energy monotonically increased with increasing iBuPOSS content, indicating a lower sensitivity of POSS copolymers to changes in temperature. The authors attributed their observations to the microscopic topology of constituent polymer chains to be altered by iBuPOSS comonomers that act as compact volumetric branches. POSS also induced significant reduction of melt viscosity in POSSMe-styrene copolymers,4 blends of POSS-sty with polystyrene (PS),45 and POSS macromers dispersed in a polymer matrix.11 However, Cheng et al.44 reported no changes in melt viscosity of POSS macromers dispersed in a polymeric matrix of poly(2vinylpyridine) (PVP). Admittedly, the influence of POSS on the thermal and viscoelastic properties, among other physical properties, is quite complex. The dynamics of polymers in the condensed state remains at the focus of current debate. The reason for this seems to be associated with the local and global dynamics of polymeric chains. The reptation model46 has provided us with the framework for understanding of transport properties in entangled linear polymers. However, recent developments in polymer chemistry enable controlled synthesis of more complex polymeric architectures where the reptation model is not satisfactory. In particular, for the complex polymeric architectures like POSS macromers it would appear that molecular interactions (physical and/or chemical) and internal degrees of freedom will play a significant role in the chain dynamics. The hybrid nature of POSS-based copolymers means that these can exhibit dual behavior: dynamics and properties of macromolecules and behavior and properties of inorganic nanoparticles. Many studies focused on high molecular weight copolymers with molecular weights at the borderline or above the entanglement molecular weight. It is believed that in the limit of high molecular weight the polymeric nature dominates the dynamics where reptation46 and “sticky” reptation47 theories would be applicable. On the other hand, in the case of low molecular weight, i.e., molecular weight smaller than the entanglement molecular weight, the dynamics would be dominated by POSS as the influence of entanglements is removed. Therefore, this paper reports the thermal and linear viscoelastic properties of a series of unentangled copolymers of POSS−styrene, denoted POSS-sty, with emphasis given to the effect of POSS concentration. For comparison purposes we also investigated the viscoelastic properties of neat polystyrene (PS). The results will show that POSS incorporation modifies substantially the thermal and rheological properties of polystyrene where cooperative interchain and/or intrachain POSS−POSS (physical) interactions give rise to molecular

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EXPERIMENTAL SECTION

Materials. Poly[(propylmethacryl-heptaisobutyl-POSS)-co-styrene] nanocomposites (Scheme 1) containing 15 wt % (abbreviated

Scheme 1. Chemical Structure of Poly[(propylmethacrylheptaisobutyl-POSS)-co-styrene] Nanocomposites (Hybrid Plastics)

POSS15-sty), 25 wt % (abbreviated POSS25-sty), and 45 wt % (abbreviated POSS45-sty) POSS were synthesized by Hybrid Plastics (Hattiesburg, MS) and purchased from Sigma-Aldrich (St. Louis, MO). Amorphous narrow molecular weight polystyrene (PS) of Mw = 13 000 g/mol was obtained from Polysciences Inc. (Warrington, PA); it is denoted PS13k. The materials were used as-received. The chemical structure of POSS-sty, as reported by Hybrid Plastics, is shown in Scheme 1. Molecular Weight Characterization. The molecular weight of POSS−styrene nanocomposites was determined by size exclusion chromatography (SEC). The polymers were dissolved in tetrahydrofuran (THF), and filtered solutions were injected into a HP 1100 series HPLC system (Agilent Technologies) equipped with an Agilent diode array detector (240 nm) and PL GPC Software from Polymer Laboratories. The universal calibration curve was obtained using monodisperse PS standards (Agilent Technologies). The results are summarized in Table 1. The rather low molecular weights indicate that the polymers are unentangled (the entanglement molecular weight of PS is ca. 17 300 g/mol) (see ref 48, p 374).

Table 1. Physical Chemical Properties of Hybrid POSS− Styrene Nanocomposites sample PS13k POSS15sty POSS25sty POSS45sty

POSS (wt %)

M̅ n (g/mol)

M̅ w/M̅ na

Rhb (nm)

D × 107 b (cm2/s)

0 15

12 400c 11 480

1.07c 1.1

5.3 ± 0.2 4.03 ± 0.05

10.0 ± 0.5 13.0 ± 0.01

25

13 400

2.0

8.33 ± 0.05

6.37 ± 0.05

45

16 700

1.9

24.6 ± 1.9

2.18 ± 0.16

a

a

Determined by gel permeation chromatography (GPC). bDetermined by dynamic light scattering (DLS). cPolymer Laboratories standard. Dynamic Light Scattering. The hydrodynamic radius, Rh, and diffusion coefficient, D, of PS and POSS-sty nanocomposites were determined by dynamic light scattering, DLS, using the Mobiuζ DLS (Wyatt Technology Corp., Santa Barbara, CA). Data analysis was carried out using the software Dynamics, v. 7.3.1.15 (Wyatt Technology). The intensity autocorrelation functions were averaged over 10 scans, and the experiments were carried out at least in triplicate. The nanocomposites were dissolved in chloroform, and measurements were performed at 25 ± 0.1 °C. The size calibration B


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The fractional free volume at the glass transition fg is related to c1g by the following equation:

was verified using bovine serum albumin (BSA) in PBS without Ca, per manufacturer’s manual. X-ray Scattering. The microstructure of the as-received nanocomposites was investigated by wide-angle X-ray scattering (WAXS). Measurements were conducted in symmetrical reflection mode on a slit collimation goniometer using a 1D scintillation counter at room temperature. X-rays from the 18 kW DMAX2500 (Rigaku Americas Corp., The Woodlands TX) rotating anode generator were passed through a monochromator in order to remove the Cu Kβ X-rays. Samples were rotated on their plane at 30 rpm in order to randomize the contributions to scattering. The software PowderX49 was used to analyze the X-ray traces and obtain plots of intensity as a function of diffraction angle 2θ. Thermal Analysis. The glass-transition temperature, Tg, was determined using a differential scanning calorimeter DSC Q200 (TA Instruments, New Castle, DE). Samples of about 5 mg were loaded in aluminum pans and heated at a rate of 20 °C/min under a nitrogen atmosphere. A first scanning was carried out from ambient temperature to 150 °C, and the samples were held for 2 min in the molten state to erase previous unknown thermal history. Then the samples were cooled down to 40 °C at 10 °C/min, and a second heating scan was carried out heating up to 150 °C at 10 °C/min. The temperature corresponding to the midpoint in the heat capacity step rise was used for the determination of Tg. The thermal decomposition temperatures, Tdec, were determined by modulated thermogravimetric analysis (MTGA), using the TGA Q500 manufactured by TA Instruments (New Castle, DE). Samples of about 20 mg were scanned from room temperature at 20 °C/min. Shear Rheometry. The viscoelastic properties were studied with the strain-controlled ARES rheometer (TA Instruments, New Castle, DE). The rheometer was equipped with 8 mm diameter parallel plates, and a gap of 0.5 mm was utilized throughout. The existence and extent of the linear viscoelastic (LVE) regime were determined measuring the dynamic storage G′(ω) and loss moduli G″(ω) as a function of strain, γ, at 1 Hz. The rheological measurements were carried out in the range 100−150 °C. The temperature control was better than ±0.1 °C. The experimental protocol consisted of heating the rheometer to a predetermined temperature, holding isothermally for 15 min to equilibrate, and then setting the gap separation. Fresh samples were loaded in the preheated rheometer, holding for 15 min to allow for thermal equilibrium before measurements started. Data Analysis. The isothermal plots of G′(ω) and G″(ω) taken over a range of temperatures T were superimposed on a reference temperature Tref = 150 °C by a translation of log aT along the frequency axis utilizing Orchestrator software (TA Instruments). No shifts along the modulus axis were required. The temperature dependence of the shift factors is described by the empirical Williams−Landel−Ferry (WLF) relationship:50

log aT =

fg /B =

m=

■

c 2ref = fref /αf

(3)

(4)

c 2g = c 2ref + Tg − Tref

(5)

f (T ) = fg + αf (T − Tg)

for T > Tg

(8)

hydrodynamic radii Rh of PS and POSS-sty nanocomposites are of the same order of magnitude, ca. 5 nm, except at 45 wt % POSS content, which is slightly larger. These results are consistent with the molecular weight determination by GPC (Table 1). Note that the size distribution of POSS25-sty is bimodal with a large molecular weight component. However, it was determined that the size biomodality rose from a contribution in molar mass of only 5%. It is therefore considered negligible and does not influence the findings of this research. Wide-angle X-ray scattering (WAXS) patterns of POSS− styrene nanocomposites are shown in Figure 2. The intensity traces exhibit two broad (in 2θ) intensity maxima denoting lack of crystalline order in the nanocomposites. The intensity maximum at lower scattering angle is associated with a mean interchain spacing. The intensity maximum at higher scattering angle arises from correlations among phenyl rings.52,53 The first intensity maximum is located at 2θ = 9.44°, 9.30°, and 8.88° for POSS15-sty, POSS25-sty, and POSS45-sty, respectively. The

c1ref c 2ref c 2ref + Tg − Tref

(7)

Figure 1. Size distribution of hybrid POSS-sty nanocomposites as determined by DLS. POSS content is (a) 0 (○), (b) 15 (△), (c) 25 (□), and (d) 45 wt % (▽).

where αf is the temperature coefficient of fractional free volume, B is a constant found by Doolittle to be of the order of unity,50 and f ref is the fractional free volume at the reference temperature Tref. The WLF parameters at the glass transition temperature Tg were calculated using the following equations:48

c1g =

c 2g

RESULTS AND DISCUSSION Physical Chemical Properties and Microstructure. The physical chemical properties of PS and POSS-sty nanocomposites are summarized in Table 1, and the size distributions obtained from intensity autocorrelation functions are shown in Figure 1. The results showed that the average

ref where cref 1 and c2 are the WLF parameters determined at the reference temperature Tref. These parameters were utilized to extract important material properties:

(2)

Tgc1g

According to the WLF theory, the temperature dependence of the fractional free volume f can be written approximately as48

(1)

c1ref = B /2. 303fref

(6)

The fragility, m, which is a parameter characterizing the sensitivity of molecular dynamics to changes in temperature, can be determined from the WLF parameters:51

− c1ref (T − Tref ) c 2ref + (T − Tref )

1 ln(10)c1g

C


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Figure 2. WAXS intensity traces of hybrid POSS-sty nanocomposites. POSS content is (a) 15, (b) 25, and (c) 45 wt %. Cu Kα radiation.

corresponding d-spacing (utilizing Bragg’s law, 2d sin(2θ/2) = λ) are d = 9.36, 9.50, and 9.95 Å, respectively. Thus, upon increasing POSS content, the d spacing increased and the corresponding reflection was slightly narrowed (in 2θ), suggesting that higher concentrations of POSS expand the intermolecular spacing, but crystalline order is not attained even at the highest POSS concentration investigated, 45 wt %. Note also the increase of intensity as POSS content increased. On the other hand, the second amorphous halo was not influenced by the presence of POSS; the maximum of intensity was located at 2θ = 19.16°, 19.30°, and 19.03°. The corresponding average d-spacing are d = 4.63, 4.60, and 4.66 Å, respectively. WAXS patterns of the POSS−-styrene nanocomposites shown in Figure 2 are similar to those obtained by Romo-Uribe et al.4 and Wu et al.,8 who studied a series of POSS−styrene copolymers. Thermal Properties. The thermal stability of the nanocomposites was determined via thermogravimetric analysis (TGA). Figure 3 shows mass loss traces as a function of temperature for the POSS-sty composites with (a) 0, (b) 15, (c) 25, and (d) 45 wt % POSS content. The results show that neat PS (Figure 3a, trace i) exhibits a 5% mass loss at Tdec = 397 °C. Increasing temperature there is rapid mass loss, and by 460 °C all material has burnt up. The nanocomposite with 15 wt % POSS content (Figure 3b, trace ii) exhibited an onset of mass loss at ca. Tdec = 360 °C, some 37 °C less than the neat PS. The significant decrease of thermal decomposition temperature Tdec is not associated with molecular weight differences. Table 2 also lists the thermal decomposition temperature of a high molecular weight PS of 400 kg/mol (denoted PS400k), showing no difference with the low molecular weight counterpart. After the onset of thermal decomposition the nanocomposite exhibited a slower rate of mass loss, as indicated by the derivative traces shown in Figure 3b, and by 490 °C all the material burnt off. Figure 3a shows that higher concentrations of POSS further reduced the onset of thermal degradation (traces iii and iv). However, the rates of mass loss continue decreasing at higher POSS content, as shown in Figure 3b. Although the POSS-sty nanocomposites have lower onset of thermal degradation temperature, the slower rates of mass loss suggest that the siloxane phase acts as “protective layer” of the organic component, thus slowing down the rate of mass loss. However, it is unclear why the nanocomposites exhibit lower thermal decomposition temperature. It is suggested that the pendant POSS groups induces flexibility to the molecular chain and acting as internal

Figure 3. (a) Mass loss as a function of temperatura of hybrid POSS− styrene nanocomposites. POSS concentration of (i) 0, (ii) 15, (iii) 25, and (iv) 45 wt %. (b) Derivative of mass loss traces in (a).

Table 2. Thermal Properties of POSS-Sty Nanocomposites sample

Tga (°C)

Cpa (J/(g °C))

Tgb (°C)

Cpb (J/(g °C))

Tdecc (°C)

PS13k PS400k POSS15-sty POSS25-sty POSS45-sty

95.6 106.0 84.6 73.4 65.9

0.307 0.243 0.282 0.213 0.162

91.4 99.8 82.6 72.5 66.2

0.197 0.234 0.291 0.215 0.177

397 398 360 337 344

a

Tg and Cp determined from second heating scan. bTg and Cp determined from cooling scan after melting the specimen. cThermal degradation temperature at 5% mass loss.

“plasticizer”, similarly to POSS copolymers with very close chemical structure investigated by Mather et al. denoted styrene-r-styryl isobutyl-POSS (iBuPOSS).8 The intramolecular freedom would be reflected in the reduction of glass transition temperature (and increase of free volume), as shown below. The modification of macromolecular dynamics impacts the glass transition temperature Tg as long-range molecular motions are activated. Figure 4 shows DSC heating scans of PS and POSS-sty nanocomposites. These traces were obtained after erasing the unknown thermomechanical history of the specimens by first melting the specimens, cooling, and then reheating at 10 °C/min. The DSC traces exhibit the stepwise increase of heat capacity associated with the glass transition. The neat polymer (Figure 4a) exhibits a Tg,bulk of 95.6 °C. Strikingly, the Tg of POSS15-sty decreased by 11 °C, relative to D


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relative to the neat polymer, ΔTg = Tg − Tg,bulk exhibits negative deviations from the glass transition temperature of the neat polymer, Tg,bulk. The ΔTg reached a minimum of ∼−35 K at 45 wt % POSS content. The negative deviations of ΔTg have been reported for unltrathin polymeric films, for model nanocomposite thin films mimicking confinement, and for other POSS-based copolymers.8,54,55 However, ΔTg of the POSS-sty nanocomposites are quite large, certainly larger than for spherical nanoparticles, i.e., silica54 and gold55 and high molecular weight iBuPOSS copolymers.8 The considerable reduction of ΔTg may be a consequence of higher degree of intramolecular freedom, an apparent contradiction given the bulky POSS groups. As the POSS macromers are attached to the styrene backbone, it is expected that the free volume would increase. The free volume will be evaluated from the linear viscoelastic properties and TTS analysis in the following section. Linear Viscoelastic Behavior. Dynamic frequency sweeps were carried out from 100 °C up to 150 °C for the neat polymer and the nanocomposites, and master curves were constructed by shifting along the frequency axis applying the time−temperature superposition (TTS) principle; the master curves at Tref = 150 °C are shown in Figure 6. No vertical shifting was necessary. The elastic and viscous moduli G′ and G′′ are plotted as a function of scaled frequency of oscillation ωaT. The neat PS (Figure 6a) exhibits a Rouse behavior, typical of unentangled polymers,48 where the terminal regime is characterized by G′ ∼ ω1, G′′ ∼ ω2, and G′ < G′′. Increasing the frequency both moduli scaled as G ∼ ω1/2. The viscoelastic spectrum of the nanocomposite with 15 wt % POSS content is shown in Figure 6b; it also shows the terminal regime where G′ < G′′. However, there are distinct differences relative to the neat polymer matrix. Note that the viscoelastic spectrum is shifted to higher frequencies (shorter relaxation times), and the viscous component G′′ dominates and is considerable larger than the neat PS. The slope of G′′ at the longest relaxation times is 1. On the other hand, the elastic shear modulus G′ does not scale with the squared frequency. Instead, as frequency decreased, G′ exhibited a change of slope at the longest relaxation time, as indicated by the arrow. The viscoelastic spectra of POSS25-sty and POSS45-sty nanocomposites exhibit the same features as POSS15-sty, as shown in Figures 6c,d. The viscoelastic response is predominantly dissipative as G′ < G′′ over the frequency range investigated, and G′′ scaled as ∼ω1. However, the viscoelastic spectra did not follow Rouse-like behavior. The most intriguing feature is the presence of a minimum in G′ (indicated by the arrow in each plot) at the long relaxation times, and this minimum shifted to higher frequencies as POSS content increased. The viscoelastic spectra clearly show the influence of the rather bulky POSS groups on the macromolecular dynamics. The fast dynamics evidenced by the viscoelastic response correlates with the glass transition temperature behavior shown in Figures 4 and 5, where Tg decreased. Figure 7 shows a plot of master curves of mechanical damping tan δ [= G′′/G′] as a function of scaled frequency of oscillation for all the nanocomposites. These spectra summarize the profound influence of POSS on the macromolecular dynamics. The neat PS (Figure 7a) shows a typical monotonic reduction of tan δ as frequency increased. On the other hand, the tan δ curves of the POSS-sty nanocomposites are quite different. First, these are shifted to larger frequencies (i.e.,

Figure 4. Heating scans of hybrid POSS−styrene nanocomposites. POSS concentration of (i) 0, (ii) 15, (iii) 25, and (iv) 45 wt %. Scans obtained after cooling from the melt.

the polymer matrix (Figure 4b). The Tg further decreased for the POSS25-sty nanocomposite (Figure 4c) about 22 °C relative to the neat polymer. Finally, Figure 4d shows that the POSS45-sty nanocomposite exhibited further decrease of Tg of ca. 35 °C. There is also a monotonic reduction of heat capacity Cp as POSS content increased, as shown in Table 2. The reduction of Tg and Cp was also reported by Mather et al.8 for iBuPOSS copolymers with slightly different chemical structure than those investigated here. However, those authors reported a Tg reduction of only 5 °C at 15% POSS content and ∼9 °C at 50 wt % POSS content. It is noted that the iBuPOSS copolymers had over 1 order of magnitude higher molecular weight than the series of our investigation. The influence of molecular weight and chemical structure on Tg will be discussed below. Figure 5 and Table 2 summarize the profound influence of POSS on the glass transition of the nanocomposites. Data for the high molecular weight PS400k are also included to evidence that the reduction of Tg induced by POSS is not a molecular weight effect. The change of glass transition temperature

Figure 5. Glass transition temperature increment ΔTg (●) and molecular weight (□) as a function of POSS concentration. Dotted lines are intended as a guide for the eye. E


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Figure 6. Master curves of POSS-sty nancomposites; dynamic shear moduli (G′,G′′) as a function of scaled frequency of oscillation. POSS concentration: (a) 0, (b) 15, (c) 25, and (d) 45 wt %. Tref = 150 °C.

frequency a maximum of tan δ follows which corresponds to the transition regime. Finally, at even higher frequencies tan δ decreases and tends to zero as the glassy regime is approached.48 The minimum of tan δ of POSS45-sty clearly evidence the buildup of elastic component G′ probably associated with long relaxation molecular interactions. The nanocomposite melts are unentangled; thus, the interactions giving rise to the minimum of tan δ must be weak and these may be steric POSS−POSS interactions. It is also possible that POSS forms nanoaggregates, especially at higher POSS content as previously observed by WAXS7 and TEM analysis reported by Mather et al.8 Then, it may be that some polymer chains may be confined by POSS nanoclusters locally slowing down the dynamics. POSS25-sty exhibits similar behavior as POSS45-sty, with tan δ first growing to an intermediate maximum and then decreasing again. However, the minimum in tan δ at the lowest frequency (highest temperature) range was not attained, at least in the temperature range investigated. POSS15-sty exhibits an intermediate viscoelastic behavior between the linear polymer melt and the POSS nanocomposites. These results reinforce the notion that the presence of POSS and certain concentration of POSS, probably above 25 wt %, is necessary to activate (weak) molecular interactions promoting an intermediate rubber-like behavior. The change of macro-

Figure 7. Master curves of mechanical damping tan δ as a function of frequency of oscillation for hybrid POSS-sty nanocomposites. POSS concentration: (a) 0, (b) 15, (c) 25, and (d) 45 wt %. Tref = 150 °C.

shorter relaxation times) and the frequency shifting is a growing function of POSS content. The most interesting feature is the production of a minima followed by a maxima as frequency increased, clearly observable for POSS45-sty. A minimum in tan δ at frequencies above the terminal regime is usually associated with the rubber-like regime. Then, continue increasing the F


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Macromolecules molecular dynamics induced by POSS covalently attached to polymer chains was first reported by Romo-Uribe et al.4 The results of Figure 7 also show that there is a reduction of energy dissipation. The maximum of mechanical damping tan δ in the “transition” regime decreased as POSS content increased. That is, the molecular interactions and nanoconfinement afforded by POSS restricted local molecular motions and therefore reduced the energy dissipation (i.e., viscous dissipation) per cycle of oscillation. The behavior in the terminal regime was further analyzed as shown in Figure 8. According to the Doi−Edwards reptation

Figure 8. Master curves of hybrid POSS-sty nanocomposites. POSS concentration: 0 (●), 15 (■), 25 (▲), and 45 wt % (▼). Tref = 150 °C.

Figure 9. (a) Shift factors for TTS analysis for (i) 0, (ii) 15, and (iii) 25, and (iv) 45 wt % POSS content. (b) Activation energy Ea as a function of POSS concentration. At 0% Ea corresponds to PS13k (■) and PS240k (□). Dotted line is only intended as a guide for the eye.

theory, the shear moduli of linear flexible polymers follow the relationship log G′ = 2 log G″ − log(ρRT /Me) + log(π 2/8)

dependent, and a more elaborated model is necessary. Nevertheless, the results are shown in Figure 9b. The neat PS13k has flow activation energy of 160 kJ/mol. Strikingly, POSS15-sty has a considerable higher Ea of ca. 194 kJ/mol. It is noted that this activation energy is of the order of a high molecular weight PS240k, as shown in Figure 9b. On the other hand, the apparent Ea is a decreasing function of POSS concentration, estimated to be ca. 166 kJ/mol for POSS45-sty nanocomposite. Therefore, as POSS increased, the nanocomposites became more temperature sensitive and required smaller configurational activation threshold. The continuous decrease of Ea as POSS content increased indicates a reduction of configurational energy barrier. This also correlates with the reduction of glass transition temperature (Figure 4). The consequences of higher viscous component G′′ in the viscoelastic spectrum and smaller flow activation energy Ea will be reflected in the melt viscosity, as discussed in the following section. In order to rationalize the reduction of Ea, it is suggested that the bulky POSS groups laterally attached to the styrene backbone increase the free volume. This is confirmed by the TTS analysis, summarized in Table 3. Therefore, at higher concentrations of POSS the macromolecules have to overcome a smaller energy barrier to reach the activated state and to achieve the corresponding configurational rearrangement. The parameters c1ref and c2ref and polymer properties determined by TTS analysis are listed in Table 3. These are

(9)

where ρ is the density, Me is the entanglement molecular weight, T is the absolute temperature, and R (8.314 J/(mol K)) is the ideal gas constant. Note that this expression is independent of molecular weight. Thus, for an ideal polymeric liquid the slope of log G′ vs log G′′ is 2 in the terminal regime, as is the case for the melt of PS13k (filled circles). On the other hand, the slopes of log G′ vs log G′′ of POSS-sty nanocomposites are smaller than 2 and gradually decay as POSS content increased, ca. 1.5 for POSS15-sty down to 1.2 for POSS45-sty. The reduction of slope with POSS content has also been reported for polymethylstyrene−POSS copolymers4 and iBuPOSS copolymers8 and attributed to POSS intermolecular (weak) interactions. The results of our investigation add support to this hypothesis. TTS Analysis and Rheological Properties. It was found that the TTS shift factors of PS and POSS15-sty, aT, obey an Arrhenius type relation, as shown in Figure 9a. However, this was not the case for POSS25-sty and POSS45-sty; the data show some scatter. Therefore, the flow activation energy Ea determined by using the relation aT = AeEa/RT, where T is the absolute temperature, A is the pre-exponential factor, and R (8.314 J/(mol K)) is the ideal gas constant, should be taken as an “apparent” activation energy for the higher POSS content nanocomposites. The noncompliance with Arrhenius-type behavior suggests that the pre-exponential factor is temperature G


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included in Figure 10. A value of fg = 0.027 was obtained for neat PS, quite close to the 0.025 value expected for amorphous polymers.48,50 Williams, Landel, and Ferry50 pointed out that an universal value of fg is consistent with the view that the glass transition is an iso-free-volume state. On the other hand, fg was found to be an increasing function of POSS concentration. Interestingly, the lower molecular weight POSS nanocomposites (POSS-sty and CyPOSS) have larger fg values than the iBuPOSS nanocomposites, at given POSS content. The dependence of free volume on temperature taken as the difference between the thermal expansion coefficients above and below the glass transition temperature50 was determined using eq 8; the results are shown in Figure 11. It can be seen

Table 3. TTS Parameters and Rheological Properties of POSS-Sty Nanocomposites

a

sample

cref 1

cref 2 (K)

fg

αf (K−1)

PS13k POSS15-sty POSS25-sty POSS45-sty

8.0 1.9 −0.36 0.98

92.9 68.2 10.9 62.7

0.0274 0.0337 6.5 −0.15

5.84 × 10−4 33.0 × 10−4 −0.111 70.4 × 10−4

Tref = 150 °C.

the fractional free volume at the glass transition fg and the temperature coefficient of fractional free volume αf. The parameters and properties listed in Table 3 show that TTS is not applied to the POSS-sty nanocomposites with higher POSS content as fg and αf have unreasonable values. It has been reported that TTS fails for hydrogen-bonding polymers, and it appears that the extent of TTS failure is related to the interaction strength; that is, failure is more evident for polymers with trimeric and quadruple hydrogenbonding groups.56,57 Failure of TTS has also been attributed to different temperature dependence of segmental and terminal relaxation times in polymers (including those with several hydroxyl groups).51 It is unclear the reason for the failure of TTS for the POSS-sty series. However, previous research on poly(methylstyrene−POSS) series denoted CyPOSS and CpPOSS,4 and iBuPOSS series8 where TTS was found to be applicable, suggests that it is related to molecular weight effects. That is, the CyPOSS and CpPOSS nanocomposites had Mw > 6 × 104 g/mol whereas the iBuPOSS nanocomposites had Mw > 18 × 104 g/mol, at least 5-fold larger than the molecular weight of POSS-sty series of this investigation. It is suggested that at high molecular weight the polymeric nature tends to dominate the chain dynamics, and TTS is applicable. However, at low molecular weight the bulky POSS macromers would dominate the chain dynamics modifying the rheological response, as indeed shown here (see Figures 6 and 7). This is only a plausible hypothesis and further investigation is required. The fractional free volume as a function of POSS content is shown in Figure 10 (see also Table 3). Data of fg as reported by Mather et al.8 and fg value extracted from data reported by Romo-Uribe etal.4 for CyPOSS nanocomposite are also

Figure 11. Fractional free volume of POSS-sty nanocomposites as a function of temperature. POSS concentration: (a) 0 (●) and (b) 15 wt % (■). (c) Data for CyPOSS 27 wt % (□).4 Dotted lines correspond to iBuPOSS nanocomposites with f(T) decreasing as POSS content increased,8 as indicated by the arrow.

that the fractional free volume of the neat PS slightly increased over the temperature range investigated. On the other hand, the nanocomposite POSS15-sty exhibited considerable increase of fractional free volume relative to the neat PS, and f(T) is strongly influenced by temperature. Data derived for the CyPOSS nanocomposite4 are also included in Figure 11. It can be seen that f(T) for CyPOSS is also an increasing function of temperature; however, the rate of growth is smaller as its αf is only 8.2 × 10−4. The data of f(T) as reported by Mather et al.8 are also included in Figure 11; the data covered the temperature range 120−180 °C. The upper dotted line denotes values for the neat polymer, and the lower dotted line denotes the range of values reported for the nanocomposites with POSS content 15−50 wt %. It can be seen that the iBuPOSS nanocomposites exhibit smaller values of f(T) than POSS-sty, and the f(T) for iBuPOSS was a decreasing function of POSS content (indicated by the arrow), opposite to POSS-sty behavior. The fragility, m, which is another important parameter characterizing the sensitivity of molecular dynamics to changes in temperature, was also determined from the WLF parameters using eq 7.58−60 Note that m determined from the TTS shift parameters corresponds essentially to a “normal mode” fragility,60 and values derived from this equation are usually smaller than those derived from the segmental relaxation regime.58−60 Nevertheless, the values determined using this

Figure 10. Fractional free volume of POSS-sty nancomposites, at the glass transition, as a function of POSS concentration (●, this work). Data for IBuPOSS8 (□) and CyPOSS (◆)4 nanocomposites are also included. H


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Figure 13. Melt viscosity of POSS-sty nanocomposites as a function of POSS concentration of (a) 0, (b) 15, (c) 25, and (d) 45 wt %. Lines correspond to the Cross model.

Figure 12. Fragility m of hybrid POSS-sty nanocomposites as a function of POSS concentration (●). Data of IBuPOSS (□) and CyPOSS (◇) nanocomposites extracted from TTS parameters, as reported by Mather et al.8 and Romo-Uribe et al.,4 respectively, are also included.

dilation effect in iBuPOSS nanocomposites where entanglement modulus was reduced as POSS content increased. The zero-shear viscosity values η0 were determined by fitting the viscosity data using the Cross rheology model62 η0 η(γ )̇ = 1 + (τγ )̇ 1 − n (11)

fragility for the neat polymer matrix is ca. 120, typical of glassy polymers.58,60 Interestingly, the fragility of POSS15-sty is quite high, ca. 430. However, Cheng et al.44 reported fragility values as high as ca. 300 for blends of POSS macromers dispersed in poly(2-vinylpyridine) (PVP). The fragility of the iBuPOSS and CyPOSS nanocomposites was determined from the TTS parameters, and data are also included in Figure 12. The fragility of iBuPOSS decreased monotonically as POSS content increased and then remained relatively constant at ca. m = 75. The CyPOSS nanocomposite has fragility ca. 100 smaller than neat PS of high and low molecular weight. In linear polymers a decrease of fragility has been correlated with increase of glass transition temperature,51,61 opposite to the behavior reported for iBuPOSS8 but consistent with CyPOSS behavior; this is further discussed below. It has also been reported that the increase of intermolecular interactions leads to a reduction of fragility as long as the backbone energy is kept constant.51 However, Sokolov et al.61 reported that segmental dynamics is slowed down (Tg increased), and the fragility m increased when polar interactions are promoted (by introduction of polar groups into the main chain). The behavior of m shown in Figure 11 reinforces the notion that some sort of interaction plays a role in the molecular dynamics retardation of POSS-sty nanocomposites. Melt Viscosity Behavior. Figure 13 shows the dynamic

where τ is a relaxation time and n is a coefficient characterizing the shear thinning behavior of the viscosity. The continuous lines in Figure 13 correspond to the best fit, and the Cross parameters are listed in Table 4. It can be seen that the fits are Table 4. Cross Parameters for POSS-Sty Nanocomposites (Tref = 150 °C) sample

η0 (Pa s)

τ (s)

n

PS13k POSS15-sty POSS25-sty POSS45-sty

464.0 96.9 51.2 24.0

4.138 × 10−2 3.196 × 10−4 0.636 × 10−4 0.3249 × 10−4

0.4068 0.3797 0.4188 0.3783

quite good. The reduction of zero-shear rate viscosity is quite dramatic, nearly 20-fold at 45 wt % POSS content. The influence of POSS on melt viscosity reduction is better appreciated by normalizing η0 by the viscosity of the neat polymer η0,bulk, as shown in Figure 14. The dramatic POSS-induced reduction of melt viscosity observed in POSS-sty nanocomposites is also featured in the high molecular weight POSS-based nanocomposites CyPOSS4 and iBuPOSS,8 and the data have also been included in Figure 14. Note that whereas the viscosity of POSS-sty is a decreasing function of POSS content, the viscosity of the higher molecular weight nanocomposites, CyPOSS and iBuPOSS, exhibits a minimum and then starts to increase at high POSS content. The decrease and then increase of melt viscosity for the high molecular weight POSS-based nanocomposites may be explained as follows: at higher POSS concentrations there is a tendency of POSS to aggregate, as suggested by the sharpening (in 2θ) of the main interchain reflection of CyPOSS and iBuPOSS, using wide-angle X-ray scattering.4,8 Hence, the eventual increase of melt viscosity exhibited by CyPOSS and iBuPOSS at high POSS content may be attributed

viscosity η* (= G′2 + G″2 /ω) as a function of scaled frequency, ωaT, for PS and POSS-sty nanocomposites. The results show considerable reduction of viscosity as POSS concentration increased; at 45 wt % POSS content the Newtonian plateau (and zero-shear viscosity η0) has decreased an order of magnitude. These results suggest that POSS acts as a molecular lubricating agent to the PS matrix, probably due to the substantial increase of free volume (see Figure 10). The reduction of melt viscosity by incorporation of POSS macromers to poly(methylstyrene) was first reported by Romo-Uribe et al.4 Furthermore, Mather et al.8 pointed out a I


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larly.4 The prospect of multiple interactions could serve to greatly magnify the total associative van der Waals force per macromer. This may lead to dramatically altered diffusion of polymer chains in which the usual reptation dynamics are modified to accommodate nondiffusive “anchors”, perhaps more than one per chain. The dynamics of polymers with complex architectures, including POSS copolymers, is challenging and remains largely unexplored. Furthermore, as POSS acts as a side group to the styrene backbone, it tends to increase the free volume, as shown by the TTS analysis, an effect also observed in CyPOSS and iBuPOSS nanocomposites (see Figures 10 and 11).8 Therefore, higher concentrations of POSS produces dilated regions (larger fg) where polymer chains would be confined and experience more degrees of freedom and faster dynamics, causing reduction of viscosity and glass transition temperature. The glass transition behavior is further discussed in the following section. Glass Transition Behavior and Cooperative Dynamics. Since the pioneering work of Romo-Uribe et al.4 on the thermal and viscoelastic behavior of POSS-based hybrid nanocomposites, evidence has grown on the POSS-induced modification of cooperative long-range molecular motions typical of the glass transition and modifications to the reptation dynamics in the molten state.4,7,8 The influence of POSS macromer content on the glass transition behavior of a series of nanocomposites is summarized in Figure 15. The change in glass transition

Figure 14. Normalized zero-shear viscosity of POSS-sty nanocomposites as a function of POSS concentration (●). Data for CyPOSS nanocomposites adapted from Romo et al.4 (▲) and for iBuPOSS nanocomposites extracted from dynamic moduli data of Mather et al.8 (□). Lines are only intended as a guide to the eye.

to entanglement and confinement effects as discussed by Romo-Uribe et al.4 The results of this investigation have shown that POSS strongly influenced the thermal and viscoelastic properties of polystyrene. This investigation focused on neat PS and nanocomposites with molecular weight smaller than the molecular weight for entanglement effects. Therefore, by removing entanglement effects, the modifications to the viscoelastic response determined in this investigation can be ascribed only to POSS−POSS interactions/associations modifying the macromolecular dynamics and the breakup of TTS. Furthermore, the unusual thermal and rheological properties induced by POSS are not restricted to low molecular weight hybrids, but the influence covers a broad range of molecular weights. The profound influence of POSS on thermal and viscoelastic properties has also been observed in high molecular weight copolymers CyPOSS and CpPOSS,4 the iBuPOSS,8 and the POSS-norbornene (CpPN and CyPN)7 in POSS macromers dispersed in a polymer matrix11,44 and in POSS15-sty hybrid blended with high molecular weight PS.45 It is proposed that theoretical models addressing dynamics retardation like the “sticky” reptation model47 and its modified version by Rubinstein et al.63 may be appropriate to explain the viscoelastic response of POSS-based amorphous nanocomposites, as originally proposed in the first rheology study of POSS hybrids by Romo-Uribe.4 Here it is pointed out that semicrystalline systems containing POSS exhibit buildup of viscosity associated with clustering of untethered POSS which act as sticky points producing higher viscosity and mechanical modulus, a mechanism proposed by Kopesky et al.11,64 The sticky reptation models have been applied to unentangled polymers which exhibit “apparently entangled” state due to H-bonding attractive forces and other molecular associations.56,57 Then the minimum in tan δ at longer times developed as POSS content increased (see Figure 6) will feature contributions from molecular interactions and associations, perhaps POSS−POSS. The nature of associations between POSS cages is as yet unclear. It has been proposed that van der Waals interactions between the seven substituent groups located in each macromer are capable of establishing multiple van der Waals contacts both intra- and intermolecu-

Figure 15. POSS-based hybrid nanocomposites glass transition temperature change ΔTg (filled symbols) and molecular weight (open symbols) as a function of POSS concentration. (a) POSS-sty (●, ○), (b) iBuPOSS (▲, △),8 (c) CyPOSS (■, □),4 and (d) CyPN (▼, ▽).7 Lines are intended as a guide for the eye.

temperature ΔTg (filled symbols) and respective molecular weight Mw (open symbols) are plotted as a function of POSS content. At first sight it can be seen that the change in glass transition temperature relative to the bulk polymer ΔTg [= Tg − Tg,bulk] can be negative or positive and it is not correlated with molecular weight. The molecular weight of the iBuPOSS series (△) is comparable to that of the CyPN series (▽) whereas their ΔTg are negative and positive, respectively. Figure 15 shows that ΔTg is negative for (a) POSS-sty (this work) (●) and (b) iBuPOSS (▲) nanocomposites.8 On the other hand, ΔTg is positive for (c) CyPOSS (■)4 and (d) CyPN (▼).7 These results clearly summarize the complex role that POSS plays in the segmental relaxation dynamics of the J


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hybrid copolymers. The summarized thermal behavior of Figure 15 suggests that the macromolecular architecture of each copolymer (and associated molecular interactions) may be driving the positive or negative deviations of ΔTg. That is the varying degrees of freedom/mobility enjoyed by the POSS macromer due to its position in the polymeric chain and by the nature of the R-groups attached to seven of its corners (see Scheme 1 and refs 4, 7, and 8) may be responsible for the modified cooperative dynamics. This is reflected in their significantly different values of fractional free volume at the glass transition fg (see Figure 10). In discussing the glass transition of POSS-based nanocomposites, it should also be kept in mind that POSS macromers are not rigid particles but have relaxations of their own according to the end groups attached to the polyhedral vertex.3,22 A recent review on the influence of POSS on segmental dynamics pointed out that when the moieties are well diluted in the polymer, POSS affect Tg as simple oligomeric components, following trends predicted by mixing laws (the authors utilized Fox’s law): the summary of results showed that POSS reduced Tg of polymers with higher Tg than theirs and increased otherwise.65

Angel Romo-Uribe: 0000-0003-0809-5908 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS The help of Dr. R. Cruz-Silva with GPC measurements is gratefully acknowledged. Thanks to the anonymous reviewers for their insightful comments, which benefited this work.

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SYMBOLS AND ABBREVIATIONS POSS-sty poly[(propylmethacryl-heptaisobutyl-POSS)-co-styrene] PS polystyrene D diffusion coefficient DLS dynamic light scattering DSC differential scanning calorimetry aT TTS shift factors cref WLF parameters at Tref i fg fractional free volume at the glass transition m fragility G′ dynamic elastic modulus G′′ dynamic viscous modulus Ea activation energy R ideal gas constant (8.314 J/(K mol)) Rh hydrodynamic radius tan δ mechanical damping [= G″/G′] Tg glass transition temperature Tref reference temperature ΔTg (= Tg − Tg,bulk) change of Tg relative to bulk polymer αf temperature coefficient of fractional free volume ω frequency of oscillation

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CONCLUSIONS The thermal and viscoelastic properties of POSS−styrene nanocomposites were investigated, varying POSS content up to 45 wt %. The influence of entanglements was removed by focusing on specimens with molecular weight below the entanglement molecular weight. Thermal analysis and shear rheometry exhibited a complex molecular dynamics, where POSS induced significant reduction of glass transition temperature, Tg, and up to 1 order of magnitude reduction of melt viscosity. The viscoelastic response did not follow Rouse-like behavior but evidenced molecular interactions, presumably POSS−POSS, producing an unexpected tan δ minimum at long relaxation times atypical of unentangled molecular chains. That is, low concentration of POSS-mers retained features similar to the neat homopolymer. However, a modification of the polymer dynamics occurred as the concentration of POSS increased, giving rise to less dissipative behavior at elevated temperatures in the terminal regime. Furthermore, there was a breakup of TTS at high POSS content. The fractional free volume increased significantly, and this appears to be a feature of POSS-based nanocomposites. This was accompanied by the increases of fragility m. Analysis of published data showed that POSS have induced increase and decrease of Tg, and this effect is decoupled from molecular weight. The macromolecular architecture and POSS chemical identity appear to drive the increase of free volume, viscosity reduction, and ultimately induces complex dynamics. Admittedly, the influence of POSS on polymer dynamics is quite complex and more research, experimental and theoretical, is needed. Finally, the ability of POSS to retard the macromolecular dynamics has been exploited in the design of thermoplastic copolymers that also behave as thermally responsive hydrogels. Furthermore, it is proposed that polymer processing can benefit from the ability of POSS nanocomposites to reduce the melt viscosity. These two topics have been addressed by the author, and the findings will be published elsewhere.

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