Evolution of chain entanglements under large amplitude oscillatory shear flow and its effect on crystallization of isotactic polypropylene

Journal Pre-proof Evolution of chain entanglements under large amplitude oscillatory shear flow and its effect on crystallization of isotactic polypropylene

Bao Wang, Dario Cavallo, Xiaoli Zhang, Bin Zhang, Jingbo Chen PII:

S0032-3861(19)30905-X

DOI:

https://doi.org/10.1016/j.polymer.2019.121899

Reference:

JPOL 121899

To appear in:

Polymer

Received Date:

02 June 2019

Accepted Date:

11 October 2019

Please cite this article as: Bao Wang, Dario Cavallo, Xiaoli Zhang, Bin Zhang, Jingbo Chen, Evolution of chain entanglements under large amplitude oscillatory shear flow and its effect on crystallization of isotactic polypropylene, Polymer (2019), https://doi.org/10.1016/j.polymer. 2019.121899

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Graphical Abstract

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Evolution of chain entanglements under large amplitude oscillatory shear flow and its effect on crystallization of isotactic polypropylene Bao Wanga, b, Dario Cavallob, Xiaoli Zhanga, Bin Zhanga,*, Jingbo Chena,** aSchool

of Materials Science & Engineering, Zhengzhou University, Zhengzhou 450002, People’s

Republic of China bDepartment

of Chemistry and Industrial Chemistry, University of Genova, via Dodecaneso, 31 -

16146 Genova, Italy

Corresponding Author E-mail: *[email protected](B. **[email protected]

Zhang)

(J. Chen)

ABSTRACT: The effect of large amplitude oscillatory shear (LAOS) flow on isotactic polypropylene (i-PP) chain entanglement-disentanglement transition and on the subsequent crystallization behavior was studied by dynamic rheology and polarized optical microscope (POM). i-PP with reduced entanglement density was generated through the application of LAOS flow at 180 °C. The re-entanglement process of disentangled

chains

was

in-situ

monitored

by

time-sweep

rheological

measurements. Interestingly, the re-entanglement kinetics was substantially slower than expectations based on the linear viscoelastic relaxation time of the fully entangled melt. Moreover, with the adjustment of shear conditions (strain amplitude, frequency and temperature), the entanglement density could be effectively modified. Disentangled chains exhibiting less topological constraints played a distinct role in the subsequent crystallization process. The nucleation density and growth rate of spherulites increased with reducing the entanglement density, resulting in a faster overall crystallization kinetics, compared to that in the fully entangled melt. It is worth to note that LAOS flow could produce a less

Journal Pre-proof disentangled melt state, and has a correspondingly weaker influence on the subsequent crystallization behavior, with respect to steady shear flow.

Introduction Entanglements between macromolecular chains, as one of the consequences of their considerable chain length, mainly affect the long range movement of molecular chains, thus playing an important role in several polymer physical processes, such as melt

flow,

mechanical

deformation

in

the

solid

state,

and

polymer

crystallization.[1-11] Since the concept of chain reptation was put forward by De Gennes,[12, 13] and further developed by Doi and Edwards,[14] many theoretical and experimental efforts have been devoted to study the role of chain entanglements on polymer viscosity,[15-17] glass transition,[18, 19] crystallization behavior, etc.[20-24] Among these studies, the effect of chain entanglements in controlling the crystallization process and the final crystalline structures was extensively investigated. [12] In recent years, several methods to obtain melt state featuring lower entanglement density have been reported. These methods include for instance the crystallization of polymer

chains

from

dilute

or

semi-dilute

solutions,[23,

25-28]

under

high-pressure,[8, 21, 29] and controlled polymer synthesis.[4, 5, 22] Bu et al.[20] prepared isotactic polystyrene with disentangled chains through a freeze-drying method: rapidly freezing the solution and subsequently removing the solvent by sublimation. They indicated that the collected particles after freeze-drying could crystallize much faster than the original polymer without treatment, both at low and high undercooling. Galeski et al.[21, 29] obtained linear polyethylene with reduced chain entanglements by melting extended-chain polyethylene, which was crystallized at elevated pressure and temperatures. They verified that the disentangled melt state produced in this way could survive for a certain time during annealing at high temperature. Rastogi et al.[4, 5, 22, 30] synthesized ultra-high polyethylene chains partially disentangled with each other, through controlled catalytic polymerization. They showed the remarkable influence of chain entanglements on the crystallization behavior and the final mechanical properties, in particular on drawability. From the point of molecular motion, when the entanglement density is large, the chain length between entangled nodes is short, which will limit the motion of chain segment. Thus, chain entanglement has a negative impact on conformational re-arrangement and

Journal Pre-proof crystallization behavior of the polymer. Molecular dynamic simulation is also an effective way to study polymer crystallization,[31-36] recently, the works carried out by Sommer et al.[37-39] successfully showed the importance of entanglement state in controlling the nucleation behavior in polymer crystallization. When polymer melts are subjected to intense processing conditions, rheological changes that result in lower melt viscosity and reduced melt elasticity take place. This shear modification is ascribed to a loss of entanglements, due to an imbalance between the entanglement dissolution by shear forces and their generation by Brownian motions.[40-43] The partial disentanglement by the action of shear flow is reversible.

[43-45]

The

fact

that

macromolecular

chains

undergo

an

entanglement-disentanglement transition when polymer melt are submitted to a single-step large deformation has been widely demonstrated.[41, 42, 46-50] It is known that polymer melts, under the influence of the oscillatory shear field during the whole crystallization process, exhibit rich ordered structures, as a result of different crystal growth processes and morphology, with respect to steady sheared melts.[51-55] However, the effect of LAOS flow on chains entanglement state has not been extensively studied so far. It is of interest to investigate the use of this deformation scheme to produce a partially disentangled melt state, and the subsequent crystallization behavior of the resulting melt, since the total applied deformation within a cycle is equal to zero. In this work, isotactic polypropylene with different chain entanglement density was generated through LAOS flow at temperatures above the nominal melting point. The effect of strain amplitude, frequency, and temperature on chain entanglements and crystallization behavior were studied using a rheometer and a Linkam CSS-450 shear device coupled with a polarized optical microscope. A brief comparison between the saturated melt states under LAOS flow and steady flow, based on chain entanglements and the following crystallization behavior, is discussed. To monitor the chain re-entanglement process, small amplitude time-sweep experiments were performed. Assuming that the elastic modulus obtained is proportional to the entanglement density in a non-equilibrated melt state, then the measured storage modulus G(t) follows the entanglement evolution in the LAOS flow modified i-PP melt.[17, 26, 30, 45]

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Experimental Section The polymer used in these experiments was a commercial Ziegler-Natta isotactic polypropylene (i-PP), produced by Yangzi Petroleum and Chemical Corp (Nanjing, China) under the commercial code of F401. The molecular characteristics are: Mw = 315 kg/mol, Mn = 83 kg/mol, MWD = 3.79, isotacticity index 0.96, melt flow index (MFI) = 3.0 g/10 min (ASTMD1238, 230 °C, and 2.16 kg). The polymer displays a peak melting point of 167 °C when heated at a rate of 10 °C/min (DSC method). Disk-shaped samples with 1 mm thickness and 25 mm diameter were prepared by vacuum compression molding at 210 °C. Oscillatory shear measurements were performed using a set of 25 mm diameter parallel-plate configuration with a sample thickness of 0.9 mm on an ARES-G2 strain controlled rotational rheometer. The disks were heated at about 30 °C/min to 210 °C, above the equilibrium melting point of i-PP crystals, and annealed for 3 min to erase any thermal and mechanical history of previous treatments, then subsequently cooled down to the selected shearing temperature. An amplitude-sweep experiment at 180 °C was performed to investigate the linear viscoelastic regime of the sample. Subsequently, large amplitude oscillatory shear (with strain amplitude equal to 100 % and a frequency of 1.0 Hz) was applied for varying time, to prepare disentangled sample at 180 °C. The raw stress data under LAOS flow, which drops first and then reaches a steady state without further decreasing, was checked for all the conditions to avoid edge fracture. For measuring the re-entanglement process, small-amplitude oscillatory shear with applied strain amplitude of 0.1 % and frequency of 1.0 Hz was applied. Moreover, frequency sweep measurements with strain amplitude of 1.0 % were performed to check whether the sample underwent a meaningful degradation during the whole procedure. Hereby, we define the initial sample as “equilibrium entangled” melt, the one subjected to LAOS flow as “disentangled” melt, although this does not imply a fully disentangled state, but just a lower entanglement density. The sample which underwent re-entanglement to the equilibrium state is defined as “fully recovered” melt. The “equilibrium entangled”, “disentangled” and “fully recovered” samples were cooled down to

Journal Pre-proof 128 °C at 10 °C/min and isothermally crystallized for one hour to follow in-situ the crystallization kinetics with the rheometer. Meanwhile, the same shear condition was mimicked with a Linkam CSS 450 shear device and the following crystallization process was investigated under polarized optical microscope at various crystallization temperatures, Tc, in the range of 132 - 142 °C. Simultaneously, micrographs were acquired by using a computer-controlled digital camera (Motic 2.0). The entanglement state of i-PP melt was adjusted by tuning the LAOS flow conditions. At 180 °C, with a total deformation time of 100 s, we used a fixed oscillatory frequency of 1.0 Hz and varied the strain amplitude from 10 to 125 %. Alternatively, the frequency was changed from 0.25 to 1.25 Hz, keeping fixed total strain amplitude of 100 %. Eventually, the evolution of chain entanglements during and after LAOS flow was studied. A selected large amplitude oscillatory shear protocol (strain amplitude 100 %, frequency 1.0 Hz and time 100 s) was applied at different melt temperatures, in order to investigate the temperature dependence of disentanglement and re-entanglement process. The detailed thermal and mechanical history adopted in these experiments is described in Figure 1.

Figure 1. Schematic diagram showing the experimental protocol.

Results and Discussion The linear viscoelastic regime at 180 °C was preliminary assessed to select the appropriate strain amplitude for the LAOS flow experiments. Figure 2 reports the

Journal Pre-proof complex viscosity as a function of strain amplitude, it can be seen that the non-linear response of deformation to the applied strain amplitude starts approximately above 30 %.

Figure 2. Amplitude sweep test for i-PP melt at 180 °C.

The evolution of the storage modulus at 180 °C during LAOS flow at fixed frequency 1.0 Hz is shown in Figure 3(a). At a given strain amplitude, the value of storage modulus decreased continuously with deformation time, until it reaches a steady value. Meanwhile, with the increase of strain amplitude, the onset time of the steady melt state decreases, which is analogous to previously reported steady shear melt modification phenomenon.[56, 57] The lower storage modulus produced during LAOS flow is explained by the alignment of the polymer chains, which decreases the number of constrains, effectively causing an entanglement-disentanglement transition up to a steady state value of entanglement density.[41, 42, 45, 47, 56, 57]

Figure 3. (a) Evolution of storage modulus during LAOS flow; (b) build-up of storage modulus with time in “disentangled” i-PP melt, G′(t) is normalized by the equilibrium modulus of “fully

Journal Pre-proof recovered” melt, 𝐺'𝑚𝑎𝑥.

As well described by Ferry,[58] the average molecular mass between chain entanglements, , is inversely proportional to the entanglement density, which is linked to the elastic modulus in the rubbery plateau regime at high frequency, 𝐺0𝑁. Therefore, 𝐺0𝑁 is a property of the entanglement network, related to the elastic response of the polymer melt, which is better described by the equation: 𝐺0𝑁 = 𝜌𝑔𝑁 𝑅𝑇/, where 𝑔𝑁 is 1 or 4/5 depending upon convention,[59] ρ the density, R the gas constant, T the absolute temperature. The elastic modulus obtained by small amplitude oscillatory shear at linear viscoelastic region is related to the entanglement density. In general, disentangled chains during heating or annealing will tend to recover the entangled state through the well-known chain-reptation process. The process of entanglement formation, however, unavoidably takes time. To gain further insight on the melt state under LAOS flow, and verify the existence of disentanglements in i-PP melt, the build-up of storage modulus of LAOS flow-induced non-equilibrium i-PP melt at 180 °C after subjected to LAOS flow with frequency 1.0 Hz, strain amplitude 100 % and varying time (0 s, 50 s, 75 s and 100 s) was shown in Figure 3(b). For a better comparison of the data at different temperatures and clear visualization of the relative entanglement density, the modulus is normalized by the equilibrium value of “fully recovered” melt. In the first stage, a relatively low storage modulus is observed for all the samples. As the disentangled chains in i-PP melt tend to re-randomize spontaneously and entanglements take place during the annealing process, an increase in modulus can be obviously observed. The modulus build-up typically takes about 10 minutes. With the increase of shear time, the characteristic modulus recovery time increases, due to higher density of disentanglements in i-PP melt. On the contrary, for the “equilibrium entangled” melt, no build-up of storage modulus during annealing can be observed in Figure 3(b). Interestingly, the modulus recovery time in these tests are in the order of 102 s, around 2 orders of magnitude longer than the linear viscoelastic relaxation time d, (≈

Journal Pre-proof 0.86 s) as obtained from the cross-over frequency between the storage modulus and the loss modulus (see Figure 4) for the re-orientation of fully entangled chain over a distance equal to its size. Similar prolonged recovery time was also found in the entanglement recovery in shear-modified polymer solution[44] and freeze-dried dilute solutions[26]. This observation suggests that, after nonlinear flow, the neighboring chains are not fully entangled and motion over distances commensurate with the chain size does not lead to full recovery, whereby the complete re-entanglement process requires cooperative chain motion over a length scale exceeding that associated with linear relaxation.[44, 60]

Figure 4. Master curves of storage modulus, G′, and loss modulus, G″, versus the frequency at the reference temperature of 180 °C for i-PP.

Moreover, the entanglement density in i-PP melt decreased substantially under the LAOS flow. For example, when LAOS flow time is 100 s, the entanglements loss, proportional to the decrease in G′, is around 1/3, as shown in Figure 3(b). On the other hand, we can exclude any meaningful sample degradation upon LAOS flow, since the equilibrium value of the storage modulus is always recovered after sufficient relaxation time as shown in Figure 5, where the frequency sweep data of fully recovered i-PP melt after LAOS flow overlap with that before LAOS flow.

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Figure 5. Frequency sweep of “equilibrium entangled” i-PP melt without flow and “fully recovered” i-PP melt after LAOS modification (frequency 1.0 Hz, strain amplitude 100 %, time 100 s) at 180 °C.

In recent years, several studies indicating that entanglements play very important role in polymer crystallization have emerged.[6, 20, 22, 23, 29] When entanglement density is large and the chain length between entangled nodes is short, the rotation of molecular chain and the motion of chain segments are limited. As such, the crystallization behavior of LAOS flow-induced partially “disentangled” i-PP melt and “equilibrium entangled” i-PP melt was compared by means of polarizing optical microscopy and rheometry, to investigate the effect of disentanglement on the crystallization behavior of i-PP. Figure 6 shows the optical micrographs of spherulites, growing during isothermal crystallization at 135 °C, from a “disentangled” and “equilibrium entangled” i-PP melt, respectively. It appears clear that the amount of nuclei in “disentangled” i-PP melt is much higher with respect to that in the fully entangled melt. This result is consistent with the reports from Hikosaka and Yamazaki that disentanglements could speed up the nucleation process effectively.[61-64]

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Figure 6. Polarized optical micrographs of “equilibrium entangled” melt (a-c) and partially “disentangled” i-PP melt (d-f) during isothermal crystallization at 135 °C for the indicated times.　 LAOS conditions to produce the disentangled state are: frequency 1.0 Hz, strain amplitude 100 % and time 100 s at shear temperature 180 °C.

Moreover, the Rheo-POM experiments allowed us to quantify the spherulitic growth rate after different LAOS flow leading to either partially “disentangled” or “equilibrium entangled” melts. Figure 7(a) reports the measured growth rates at different isothermal crystallization temperatures, Tc, both for “disentangled” and “equilibrium entangled” i-PP melt. A small but meaningful increase in the growth rate of spherulites for “disentangled” i-PP with respect to that of entangled i-PP is observed, which is attributed to the lower number of entanglements per chain. However, the growth rate of partially “disentangled” and “equilibrium entangled” i-PP becomes similar at higher crystallization temperatures, since the crystallization process is slow at high temperature and obstacles for transport of macromolecules due to entanglements is less important than nuclei formation on the growing crystal face. Galeski and Pawlak also observed similar crystallization behavior of i-PP with disentanglements.[10, 23, 65] As disentanglement can be preserved in the melt for a certain time and the spherulites growth is sensitive to entanglement density in the melt, it may be feasible to trace the re-entanglement process during annealing at high temperature by measuring the growth rate. Figure 7(b) shows the growth rates of

Journal Pre-proof spherulites from “disentangled” i-PP melt which was annealed at 180 °C for different times after the LAOS flow and before cooling to the crystallization temperature. The growth rate of “disentangled” i-PP is initially almost 30 % higher than that of “equilibrium entangled” melt. However, the value decreases with holding time at 180 °C, and reaches a plateau equal to the equilibrium value when the annealing time is above 20 min. Such an effect is analogous to what observed by Galeski and Pawlak.[10, 23, 65] The time needed for the recovery of entanglements in “disentangled” i-PP melt determined by tracking the changes in the growth rate is in very good agreement with that obtained on the basis of storage modulus build-up in the rheological study.

Figure 7. (a) Growth rate of spherulites for “equilibrium entangled” and partially “disentangled” i-PP melt at different temperatures, Tc; (b) growth rate of spherulites at 135 °C after different annealing times at 180 °C, both for “equilibrium entangled” and partially “disentangled” i-PP, the value of rate is normalized by the growth rate, G0, of “equilibrium entangled” i-PP melt cooled down without annealing.

In addition, the overall isothermal crystallization kinetics for the “equilibrium entangled”, partially “disentangled”, and “fully recovered” i-PP melts can also be measured by means of SAOS rheological measurements at the crystallization temperature. Indeed, during crystallization, the storage modulus increases up to a steady state, in parallel with the increase of the crystalline fraction in the sample.[66, 67] As can be seen from Figure 8, the crystallization kinetics of “disentangled” i-PP melt is much faster than that of “equilibrium entangled” melt, as a result of the proved

Journal Pre-proof higher amount of nuclei and growth rates discussed above. However, when the disentangled chains have relaxed through Brownian motion, and the melt has fully recovered to equilibrium state, the crystallization kinetics revert back and overlapped with that of original “equilibrium entangled” melt.

Figure 8. (a) Build-up of storage modulus with time at 180 ºC in “disentangled” i-PP melt, prepared by the application of LASO ( frequency 1.0 Hz and strain amplitude 100 % for 100 s) flow, G′(t) is normalized by the equilibrium modulus of “fully recovered” melt, 𝐺'𝑚𝑎𝑥;(b) Evolution of the elastic modulus during isothermal crystallization at 128 °C for “equilibrium entangled” (no shear), “partially disentangled” (holding for 10 s and 200 s at 180 C after LAOS flow) and “fully recovered”(holding for 700 s) i-PP melts.

Similarly to the phenomenon observed under steady shear of polyethylene,[56, 57] i-PP undergoes a partial entanglement-disentanglement transition to a state with lower entanglement density because of the alignment of molecular chains under LAOS flow.[55, 68] This state features a higher mobility of i-PP chains and consequently a faster crystallization kinetic, compared to that in the “equilibrium entangled” melt. However, during the annealing process at temperature higher than the nominal melting point of i-PP, the disentangled chains diffuse toward each other spontaneously and the entanglement density increases again, hindering the crystallization behavior of i-PP melt. When the disentangled chains fully reorganize to the equilibrium state, the crystallization behavior of the fully relaxed melt return equal to the one of the melt never submitted to LAOS flow. Flow-induced crystallization has drawn much attention in the past.[69, 70] The

Journal Pre-proof statement that flow promotes local parallel arrangement of chain segments and generates metastable denser phase with an intermediate degree of order between crystalline and amorphous phase, has attained a wide consensus.[69, 71, 72] However, the molecular mechanism involved in the formation of “nucleation precursors” is still unknown. The structures of shear induced precursor seem to be intimately related with entanglements. Whether flow-induced nucleation precursors play a role in the acceleration of crystallization observed for LAOS flow-induced disentangled melts will be discussed in a future work. Finally, a brief overview on the effects of oscillatory shear strain amplitude, frequency, and temperature on chain entanglement in i-PP melt is presented in Figure 9. At 180 ºC, when oscillatory shear frequency is 1.0 Hz and shearing time is 100 s, increasing strain amplitude results in progressively lower initial normalized storage modulus and longer recovery time after LAOS flow, indicating lower entanglement density after shear. However, the chain entanglement network changes only slightly when oscillatory shear strain amplitudes are in the viscoelastic region (0 - 30 %). The same trend is found for frequency in Figure 10(b). Interestingly, when the frequency is lower than the cross-over frequency of 1.0 Hz, chain entanglement-disentanglement transition still occurs, suggesting that the re-entanglement process does not simply involve chain reptation from a deformed chain network, but is instead a more complicated process. In general, at fixed temperature, higher strain amplitudes and frequencies favor the orientation of molecular chains, thus accelerating the disentanglement of i-PP chains.

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Figure 9. Modulus build-up of i-PP melts subjected to LAOS flow with different conditions. (a) 180 ºC, frequency 1.0 Hz and time 100 s; (b) 180 ºC, strain amplitude 100 % and time 100 s; (c) frequency 1.0 Hz, strain amplitude 100 % and time 100 s.

However, for LAOS (strain amplitude 100 % and frequency 1.0 Hz, applied for 100 s) flow, with the increase of shear temperature, the normalized storage modulus immediately after LAOS flow increases, while the recovery time to equilibrium decreases. This indicates a higher entanglement density in i-PP melts sheared at higher temperatures. Because chains motions in i-PP melt at high temperature are faster than that at low temperatures, so is the re-entanglement rate, despite not being exactly described by chain diffusion. We can however see that at 170 ºC, the initial storage modulus immediately after shear is higher than expected from the trend followed by the higher shearing temperatures. We speculate that this is due to the creation of special chain interactions at low shearing temperatures, i.e., LAOS flow induced nucleation precursors, which will be discussed in details in a future work. The difference between LAOS flow and steady flow on the subsequent crystallization behavior are seldom considered in the literature. [73] Considering that

Journal Pre-proof all the parameters for LAOS flow (frequency, strain, time) and steady flow (shear rate, time) affect the crystallization behavior, it is not easy to obtain a meaningful comparison based on the judgment of single variables. As

aforementioned,

under

LAOS

flow

i-PP

melt

undergoes

entanglement-disentanglement transition, and reaches a final steady state with constant entanglement density and chain conformation, as shown in Figure 3(a) and by the following recovery process in Figure 9 (a) and (b), where the re-entanglement curves overlap when the melt is in a steady state. Under steady flow, a similar entanglement-disentanglement process can be observed, and a final partially disentangled melt state is reached. As shown by Martins et al. [56], a plateau value of viscosity is reached after a certain flow time, corresponding to a decrease in the entanglement density to a content of approximately 1/3 of the equilibrium value. It is worth to note that increasing the shear rate accelerates the time to attain the final melt state, but has no influence on the achieved entanglement density. As comparison, the recovery process of “steady-state” disentangled i-PP melts under LAOS or steady shear flow at 180 ºC are shown in Figure 10 (a). For a meaningful comparison between the two shear protocols, the same average shear rate of 4 s-1 was applied for both LAOS and steady flow, for a time sufficient to reach the final partially disentangled state. Therefore, the “steady-state” disentangled melts are prepared under LAOS flow (frequency 1.0 Hz, strain amplitude 100 %, time 100 s) and steady flow (shear rate 4 s-1, and shear time 150 s). It is clear that, when i-PP melts reach the “disentangled steady state” under LAOS, it requires shorter time to completely restore to equilibrium, with respect to the steady state reached under steady flow. Meanwhile, the crystallization kinetics from “steady state disentangled” i-PP melt produced by LAOS flow is slower than that of the meal disentangled by steady shear flow (Figure 10 (b)). Both these observations point towards a lower degree of disentanglement of i-PP chains reached under LAOS flow. In general, compared to steady flow, LAOS flow is a relatively mild way to adjust entanglements in i-PP melt and has a correspondingly weaker influence on the subsequent crystallization behavior.

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Figure 10. (a) Modulus build-up of “steady state”, disentangled i-PP melts under LAOS flow (black line) and steady flow (red line) at 180 ºC; (b) Evolution of storage modulus during isothermal crystallization at 128 °C from “steady state” disentangled melts under LAOS (black line) and steady flow (red line), respectively. The dotted line represents the crystallization process of “equilibrium entangled” i-PP melt without flow.

Conclusions In this work, partially “disentangled” i-PP was generated through the application of large amplitude oscillatory shear, which was verified by the lower modulus after shear and the following modulus build-up process. Disentangled chains could be preserved for a period of time longer than the longest relaxation time at temperatures higher than the nominal melting point of i-PP. At fixed temperature, increased strain amplitude or frequency is in favor of the orientation of molecular chains, thus accelerating the disentanglement of i-PP chains. Meanwhile, the re-entanglement rate via thermally activated chain motion becomes faster with the increase of temperature. Rheological measurements assured that there was no degradation in this procedure. Crystallization kinetics of the LAOS flow modified ‘‘disentangled’’ i-PP was faster than that of well entangled i-PP due to the higher nucleation density and faster growth rates of spherulites. Moreover, compared to steady flow, LAOS flow is a relatively mild way to adjust entanglements in i-PP melt and thus has a weaker influence on the crystallization behavior. Therefore, LAOS flow offers a new and simple procedure to produce non-equilibrium disentangled polymer melts, and could be exploited in the

Journal Pre-proof future to gain more information on the effect of entanglement state on polymer crystallization.

Acknowledgements: The authors are grateful to the National Science Foundation of China (No. 11372284, 51773182, 11872338 and U1804144.)

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Journal Pre-proof 1. LAOS flow could generate disentanglements in i-PP melt. 2. Disentanglements could preserve longer time than typical reptation time. 3. The entanglement density could be modified by adjusting the flow conditions. 4. Disentanglements could accelerate the overall crystallization kinetics.