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Rapid Fabrication of Micro Structure on Polypropylene by Plate to Plate Isothermal Hot Embossing Method.

INTRODUCTION

Hot embossing is one of the most promising methods to fabricate high-precision and high-quality structures on thermoplastic polymer substrates in micro-/nanoscale [1-4]. In the early 1970s, the embryonic form of micro hot embossing was proposed by Urich et al. [5]. Microgrooves were fabricated on poly(methyl methacrylate) (PMMA) substrates using glass fibers as mold. Then in 1995, Chou S.Y. of Princeton University created vias and trenches with a minimum size of 25 nm onto thermoplastic polymer substrates [6]. His research extended the hot embossing from microscale to nanoscale.

Up to now, three different molding principles of micro hot embossing technology including Plate to Plate (P2P) [7, 8], Roll to Plate (R2P) [9, 10], and Roll to Roll (R2R) [10-13] have been successively developed to meet the increasing demand for large-area patterned polymeric substrates [14]. Among these technologies, the P2P method has advantages of high precision, good replication, flexibility in material choice, and process controllability. Many potential applications, such as microneedles, diffuser plates, microfluidics, antireflection products, and so forth, have also been developed [15-19]. As shown in Figure 1, a typical micro hot embossing process is composed of five major steps [20, 21]: (a) positioning of polymer sheet, (b) heating of a semi-finished product, a thin polymer foil, to molding temperature, followed by (c) an isothermal molding by embossing (velocity- and force-controlled), (d) the cooling of the molded part to demolding temperature, with the force being maintained, and finally, (e) demolding of the component by opening the tool.

An inherent problem of long cycle time of conventional micro hot embossing was mainly ascribed to periodically heating and cooling of embossing mold with high thermal inertia. The whole cycle time of hot embossing used to be more than 10 min [22]. Owing to its long cycle time, hot embossing method did not show competitiveness in mass production on cost and efficiency compared with common processing methods such as microinjection molding. To cut down the cycle time of hot embossing method, researchers around the world had made great efforts on process optimization and device improvement in recent years. Although these efforts had led to significant progress in different aspects, there were still some unsolved defects left.

The most usual way to reduce the cycle time of hot embossing was making the heating and cooling process much faster. T. E. Kimerling and Donggang Yao investigated a unique structured embossing tool with rapid heating and cooling capability. Different miniaturized features including microsquare and hexagonal wells, microcircular holes, and submicron surface features were successfully fabricated with a total embossing cycle time around 20 s [23]. Grewell et al. studied the feasibility of ultrasonic heating and obtained good replication while heating and cooling time as short as 10 s [24]. It was found that ultrasonic could formed a melting layer on polymer surface in several seconds while the temperature of inner part remained low. Liu S. J. proposed an infrared hot embossing process and polymeric micro-block arrays with a dimension of 100 x 80 x 40 [micro]m were successfully fabricated in a shorter cycle time [25]. Pengcheng Xie and James Lee presented a rapid hot micro embossing technique utilizing micro-patterned silicon stampers with a carbide-bonded graphene coating layer of about 45 nm thickness to implement rapid heating and cooling and the cycle time was shorter than 25 s [26].

Many efforts had also been made to optimize the parameters and devices of hot embossing process including choose lower mold temperature, much closer to the glass transition temperature ([T.sub.g]) for amorphous polymers. Worgull et al. performed a series of research to meet the requirements increasing embossing surface area and a simultaneously decreasing structure size [27]. Optimization of hot embossing process was also researched by Yong He et al. and well replicated microrectangular structure with high aspect ratio of 4 was obtained [28]. They further built a simplified hot embossing machine and best imprint quality for micro linear arrays on PMMA was achieved with a mold temperature of 150[degrees]C [29]. Wiriyakun et al. presented a two-step hot embossing method to produce cross-shape microchannels with mold temperatures of 180[degrees]C and 125[degrees]C, respectively [30]. All these studied above presented the great progress in the field of hot embossing method in recent years, but there were still some problems unsolved.

It was found that more than 90% cycle time was consumed for heating and cooling the mold together with the polymeric substrate in conventional hot embossing process. To achieve free heating and cooling of the embossing mold, a new type of Plate to Plate isothermal hot embossing method (P2P IHE method) was proposed and discussed in this article. In our previous work [31, 32], we focused on the forming process and mechanism of isothermal hot embossing for PMMA, a typical kind of amorphous polymer. Although semicrystalline polymers also showed great potential in functional microstructure devices and systems, few publications had paid attention on the micro fabrication process for them. So in this study, polypropylene (PP) was chosen as an example to explore the feasibility and mechanism of P2P IHE process for semicrystalline polymers.

It should be pointed out that the "isothermal" in our P2P IHE method represented a constant mold temperature during whole embossing process, no periodically heating and cooling steps needed. With a reasonably selected mold temperature, which was much lower than that commonly used before, demolding step can be performed once after the embossing process was done. Only the increase of substrate temperature up to the setting mold temperature occurred in this isothermal hot embossing process. Based on the constant and low mold temperature, the cycle time of P2P IHE for PP substrates could be sharply reduced to 20 s.

FUNDAMENTAL OF P2P ISOTHERMAL HOT EMBOSSING FOR SEMICRYSTALLINE POLYMERS

Theoretically, the P2P IHE method can be applied to both amorphous polymer and crystalline polymer. However, the deformation mechanisms of these two sorts of materials were totally different.

For amorphous polymers, the mechanical property and workability will meet a significant variation of several orders of magnitude around [T.sub.g]. It was also the underlying reason for the application of high molding and low demolding temperature in conventional hot embossing method. To find a compromise between constant mold temperature at a low level and the workability of amorphous polymer, the processing window of P2P IHE method existed only in a very narrow range around [T.sub.g] [31, 32].

For semicrystalline polymers--e.g., PP--crystalline region was coexisted with amorphous region when the temperature was lower than its crystalline melting point. If the mold temperature was close to or even higher than the melting temperature ([T.sub.m]), the crystalline region will be destroyed and the elementary unit of deformation turned into whole molecular chain. The deformation ability was pretty high in this situation, while the stability of embossed microstructure was poor. Therefore, the mold temperature needed to be cooled down to promote recrystallization for geometry stability. That would lead to periodical heating and cooling in conventional hot embossing process and long cycle time. In our P2P IHE process, the mold temperature was set at a constant value that far less than [T.sub.m]. The realization of small deformation on PP substrates in this situation mainly depended on the deformation ability of crystalline region, while the deformation of amorphous region was conditioned by the former. The influence of crystalline region on hot embossing process was a double-edge sword. On one hand, the force required for embossing will increase with increasing crystallinity. On the other hand, force-induced recrystallization will also enhance the geometry stability of microstructures obtained. Hence the processing window of semicrystalline polymers (e.g., PP) was much wider than that of amorphous polymers (e.g., PMMA).

It was known that the hot embossing of amorphous polymers, such as PMMA, could be carried out at high elastic state, that is, temperature between [T.sub.g] and [T.sub.f]. Unlike amorphous polymers, the semicrystalline polymers, such as PP, were almost impossible to be embossed at high elastic state, because it was a very narrow range, within 3[degrees]C to 5[degrees]C just below the crystalline melt point Tm. PP will fall into solid-like state or viscous flow state once its temperature went a little lower or higher than [T.sub.m]. Considering the existence of certain amount of amorphous region in PP, hot embossing process for PP at solid-like state with temperature above [T.sub.g] had certain feasibility. In fact, there were a lot of researches focused on processing and modification of semicrystalline polymers at solid state. Such as solid phase die drawing technology was successfully applied in the preparation of oriented PP pipes and strengthened PP ribbons by Phil Coates et al. [33-35]. The difference between hot embossing and solid phase die drawing was the dimension of product. Hot embossing process was in high precision from nano- to microscale, while the die drawing process was carried out in macroscale.

In this part, the differences between internal mechanisms of conventional hot embossing method and P2P IHE method for PP were analyzed. The change of crystallinity and crystal form of PP in P2P IHE process was emphatically analyzed. Furthermore, the appropriate processing window for P2P IHE method was distinguished with the help of stress-relaxation curves of PP under different temperatures.

Conventional Hot Embossing Method

As shown in Fig. 2, the processing window of conventional micro hot embossing for semicrystalline polymers was located in the region of melting area [22]. The shear modulus of semicrystalline polymers decreased rapidly with the increase of temperature near [T.sub.m]. From the point of filling in cavities and stress relaxation, low shear modulus and high temperature seem to be favorable because whole-chain segments of macromolecular could easily overcame interaction and entanglement between each other and occurred relative displacement of macromolecular barycenter. But, unfortunately, the flow front of the melt was not stable enough and the melting experience made PP much easier to adhere to the mold cavities, which may lead to low replication precision and defects. Another problem for conventional hot embossing method was its long cycle time of replication up to 10 min due to the needs of heating up and cooling down of the mold together with PP substrates.

The P2P IHE method for semicrystalline polymers, characterized by constant and low mold temperature during whole process, was proposed to shorten the cycle time of conventional hot embossing. Compared with conventional hot embossing method, periodical heating and cooling process of embossing mold was entirely cut out in P2P IHE method for PP.

New Type of P2P Isothermal Hot Embossing Method for PP

To shorten the cycle time of hot embossing process, the P2P IHE method was proposed. Procedure diagram of P2P IHE for semicrystalline polymers was shown in Fig. 3. Main difference between P2P IHE and conventional hot embossing was in their thermal pattern. The mold temperature for P2P IHE process was maintained at a setting value during whole embossing cycle, no heating or cooling of the mold was required. In this way, the cycle time of P2P IHE process could be reduced to about 20 s, only 1/30 that of in conventional hot embossing process.

As shown in Fig. 3, the mold temperature kept constant throughout, embossing and demolding occurred at the same mold temperature. The polymer started to recover immediately after the substrate was separated from the mold if the mold temperature was too high, as there was no cooling phase to "freeze" the microstructure [22]. So the most important parameter of P2P IHE process was the temperature of mold, which should be reasonably determined.

It can be seen in Fig. 3 that the region above [T.sub.g] and below [T.sub.m] was chosen as the molding window of P2P IHE method for semicrystalline polymers. Two fundamental factors should be taken into consideration in estimating press and thermal pattern of P2P IHE. First, a sufficient stress beyond the yield limit of PP should be applied to the substrates inducing forced high elastic deformation to fill in the mold cavities. Second, the thermal pattern should insure that majority of inner stress induced by embossing process could be relaxed before demolding of the embossed product and no detectible changes in shape and dimension of the embossed microstructures can be demonstrated as a result of creep. From the point of high replication, the mold temperature should be chosen as low as possible because lower temperature of the demolding product had a relatively low speed of creep and easy to be cooled to ambient temperature. But too low mold temperature will result in very high press and high residual stress in products. Based on an overall consideration of above factors, the mold temperature could be ranged from 105[degrees]C to 125[degrees]C for PP.

During whole process of P2P IHE of semicrystalline polymer, the embossing and demolding steps occurred with same mold temperature, determined in the temperature window of 105[degrees]C to 125[degrees]C for PP. The existence of crystalline region in semicrystalline polymer made the demolding process much easier than that of amorphous polymers, such as PMMA. Dirckx et al. had reported their research about demolding process of PMMA substrates close to [T.sub.g] with little degradation of quality [36].

Furthermore, it was supposed that orientation and recrystallization, which led to an enhancement of material property and dimension stability, will take place during the embossing process. Figure 4 displayed the wide-angle X-ray diffraction (WAXD) patterns in the range of 2[theta] = 10-50[degrees] of the PP substrate before and after embossing. The samples before and after embossing both exhibited diffraction peaks at 2[theta] = 14.2[degrees], 16.1[degrees], 17.0[degrees], and 18.7[degrees]. The peaks at 2[theta] = 14.2[degrees], 17.0[degrees], and 18.7[degrees] belonged to a crystal form (110), (040), and (130) while the peak at 2[theta] = 16.1[degrees] belonged to [beta] crystal form (300) as reported previously [37-39]. It can be seen that the diffraction peak position of PP had no change; however, there were significantly differences in intensity. Because the intensity of the diffraction peak represented the degree of the order in the material, including crystallization and orientation, the results indicated that PP was oriented and recrystallized during the embossing process. The variation of PP under this temperature was a complex combination of deformation, orientation, relaxation, and recrystallization.

To verify the correctness of above conclusions, the crystallinity and grain size of PP before and after embossed were calculated by JADE software. The crystallinity of PP increased from 51.61% to 63.37% after embossed, while the grain size decreased from 195 [Angstrom] to 130 [Angstrom], indicated that slipping and rupture of the lamellar in the spherulite occurred during the P2P IHE process of PP and a recrystallization and orientation of PP molecules had been formed.

The recrystallization phenomenon of PP induced by stress and maintained temperature made a contribution on geometry stability, while the geometry stability of embossed microstructures also depended on relaxation and creep behaviors of the polymer. Stress-relaxation curves of PP under 50[degrees]C, 105[degrees]C, and 115[degrees]C were tested using DMTA V produced by Rheometric Scientific[TM] with a 0.5% strain. As shown in Fig. 5, when the processing temperature rose up, the residual stress in the embossed product will be relaxed faster and faster during embossing cycle. The residual stress of PP at 50[degrees]C decreased from 7.97 to 7.09 x [10.sup.8] Pa in the first 20 s, 88.96% of initial stress was still remained in the sample. It is certain that so much high level of residual stress could not guarantee stable of the embossed microstructure and a higher mold temperature should be chosen. When the mold temperature is increased to 105[degrees]C, the residual stress reduced to 53.55% of initial value (from 9.71 to 5.20 x [10.sup.6] Pa) within 20 s. In contrast with the situation at 50[degrees]C, the internal stress had two order of magnitudes decrease when the mold temperature increased to 105[degrees]C. An even faster decline of residual stress from 7.64 to 2.88 x [10.sup.6] Pa, a 62.30% relaxation, can be achieved at a mold temperature of 115[degrees]C. It can be concluded that with the increase of mold temperature from 50[degrees]C to 105[degrees]C, the relaxation rate increased 5.64 times. Experiment indicated that the microstructure cannot be fabricated at a mold temperature of 50[degrees]C in 20 s. A relatively large residual stress of 5.20 x [10.sup.6] Pa was remained in the substrates, embossed at a mold temperature of 105[degrees]C, which may show an influence on geometric stability of the fabricated microstructure to a certain extent. According to our experiment, the appropriate mold temperature of 115[degrees]C and embossing time of 20 s were available for PP substrates. The embossed substrates should be cooled as quickly as possible to room temperature once they were taken out from the mold. Owing to the strong restricting effect of crystalline structure on amorphous region, the residual stress in embossed PP substrates was hard to relax, that the geometric stability of the embossed microstructure would keep well.

The theory above laid a solid foundation for P2P IHE method and explained the reason why we dared to choose this low temperature as the molding window. Low temperature also brought poor adhesion between molds and polymer substrates, which led to higher replication accuracy. For semicrystalline polymers processing by P2P IHE method, as PP, the polymeric substrate can be placed in the embossing mold under ambient temperature without preheating. During embossing period, the substrate will be heated up rapidly from ambient temperature to mold temperature underwent together with the pressing process. After the molding process finished, demolding process can be done at once and came to another embossing cycle. With the help of P2P IHE method, we successfully reduced the cycle time from no less than 10 min to 20 s.

EXPERIMENTAL

Experimental Device

The experimental hot embossing device was designed and setup. By controlling the stopping torque of the servo motor, the pressure of embossing can be acquired up to 50 kN with a control accuracy of [+ or -]50 N. The temperature of the molds was controlled by a mold temperature controller from ambient temperature to 170[degrees]C with a control accuracy of [+ or -]1[degrees]C. The moving speed of the pressing platen with the up-mold could be set from 0.5 to 5.0 m/s at opening stroke and closing stroke before the embossing mold approaches to the polymeric substrate, from that moment the pressing platen with the up-mold will be sharply slowed down to available pressing speed depending on servo motor setting and stop when the press force reaches the setting value. A PLC control system was used to achieve an accurate and intelligent control of the P2P IHE process. The effective area of embossing is 160 X 80 mm.

Mold

The embossing mold comprised up-mold and down-mold. Down-mold was a flat plate while up-mold was a plate with V-cut microstructures. They were both fixed on the adapter plate with heating oil lanes. As shown in Fig. 6, the distance between two peeks of the V-cut was 49 pm and the included angle was 90.727[degrees]. The oil lanes of the two half molds were connected in series by high temperature hoses and the hot oil from the mold temperature controller circulated in the lanes to keep constant temperature of the molds.

Image Characterization

Characterization of the microcavities of the embossing mold and microstructure of the embossed substrates were carried on by the JTVMS-1510T 3D Image Measurement System, product of China Gongguan Janten Instrument Co., Ltd. The Image Measurement System has an object field of 30-230X and digital resolution of 1 [micro]m.

Experiment Material

The sheet of GEHR PP[R] with the width of 100 mm and 0.25 mm in thickness was selected as the substrate of the experiments. Main properties of GEHR PP[R] were listed in Table 1.

Experimental Procedure of P2P Isothermal Hot Embossing

The flow chart below (Fig. 7) showed the procedure process of the new type of P2P IHE of PP substrate:

1. Start the equipment and set the mold temperature to a suitable value.

2. The oil temperature controller was circulated at least 20 min after reaching setting temperature.

3. The PP substrate with room temperature was placed to the mold and pressed for 10-60 s. Because the mold temperature was set from 105 to 125[degrees]C for PP and no phase transition occurred in this range.

4. The mold was opened and the product was taken out.

5. The next embossing cycle can start immediately after taking off previous embossed substrate, benefiting from constant mold temperature pattern.

It can be seen from Fig. 7 that the mold temperature of P2P IHE kept constant no matter how many cycles were performed and temperature of the PP substrate varied from room temperature to mold temperature in process of embossing and was cooled from mold temperature to room temperature after demolding.

RESULTS AND DISCUSSION

Parameter Optimization of P2P Isothermal Hot Embossing for PP

A set of embossing tests with identical pressure and holding time were made to qualitatively examine the feasibility of P2P IHE method. The pressure of 10 MPa was chosen and a holding time of 60 s was fixed. The mold temperature was selected as 105[degrees]C, 110[degrees]C, 115[degrees]C, and 125[degrees]C and kept constant in whole embossing process while PP substrates were set to room temperature. Demolding defects occurred frequently if mold temperature was higher than 125[degrees]C due to the stickiness of polymer. Embossing parameters for different samples were listed in Table 2.

The images of the embossed samples prepared according to the conditions in Table 2 were shown in Fig. 8. The ratio of L/[L.sub.0], H/[H.sub.0], and A/[A.sub.0] were used to evaluate the replication feature of the samples, where [L.sub.0], [H.sub.0], and [A.sub.0] represented the width, height, and angle of the V-cut in the mold, and L, H, and A represented the width, height, and angle of the V-cut of embossed samples. The least value of [L.sub.0], [H.sub.0], and [A.sub.0] was defined as the diagnostic replication rate, which may serve as a criterion of the replication rate. Main test results of the embossed samples were listed in Table 3.

Some valuable results can be concluded from Fig. 8 and Table 3. Sample 2 and Sample 3 showed excellent replication precision as their diagnostic replication rate both fell into relative accurate range of (100 [+ or -]10)%. Hence the range from 110[degrees]C to 115[degrees]C can be defined as appropriate mold temperature for PP by P2P IHE method. When the mold temperature went higher, PP could not fill the mold cavities perfectly with relatively high modulus. The deformation recovery of PP with a mold temperature higher than 115[degrees]C will become a significant factor for replication precision. It can be seen from sample 4 in Fig. 8 that the sharp corner of patterned microstructure turned into rounded corner as a products defect due to recovery. In short, the replication precision of P2P IHE for PP substrate depended on the deformation both during and after the embossing process.

Furthermore, a series of tests with different conditions were carried out to ascertain the influence of pressure and holding time on replication precision. The detailed parameters were listed in Table 4.

The images of the embossed samples prepared according to the parameters in Table 4 were shown in Fig. 9.

The results of P2P IHE from Sample 5 to Sample 10 were summarized in Table 5.

Based on an overall consideration of the sample images showed in Fig. 9 and the diagnostic replication rate in Table 5, Sample 7 and Sample 10 showed good replication precision with relative accurate range of (100 [+ or -] 10)%, while Sample 5 had an acceptable precision of (100 [+ or -] 20)%. It turns out that relatively high precise replication for PP sample by P2P IHE method can be achieved with pressure and holding time no less than 5 Mpa and 20 s, respectively. Hence, the appropriate parameters of P2P IHE for PP should be the combination of mold temperature ranged from 110[degrees]C to 115[degrees]C, pressure higher than 5 Mpa, and holding time more than 20 s. Furthermore, an overhead view of the PP sample with high replication precision was shown in Fig. 10 to detect the whole uniformity of microstructures on PP substrates. It can be seen from Fig. 10 that the micro V-cuts with a width of 49 [micro]m were neatly arranged and no molding defects (e.g., round corners and platforms) were found.

In sum, relatively short embossing cycle time of 20 s can be achieved with constant and low mold temperature, 40[degrees]C to 60[degrees]C below [T.sub.m], in whole embossing cycle. It was a broad range which should be carefully determined based on the specific properties of the PP substrates used in experiments. Different molecular chain structure (PP-H, PP-B, and PP-R), different molecular weight, different degree of crystallinity and even different brands of PP will lead to entirely different optimal mold temperature. However, it will kept in the range of 40[degrees]C to 60[degrees]C below [T.sub.m] for the majority species. For example, [T.sub.m] of the PP substrates used in the experiments listed above was measured to be 153.55[degrees]C with the method of differential scanning calorimetry (DSC) and the optimal mold temperature was finally confirmed as 115[degrees]C, approximately 40[degrees]C lower than its [T.sub.m].

To further research about the stability of microstructure on PP substrate, we cut the product into two parts along the vertical direction of micro V-cuts. One part was observed immediately to measurement the feature sizes of micro V-cuts, another part was put into the oven with a setting temperature of 110[degrees]C for 3 h before measuring. It can be seen from Fig. 11 that feature sizes of microstructures on the two parts were almost the same, which meant no obvious deformation resilience occurred after postprocessing.

An Application Example of PP Microstructure Product Made by P2P IHE Method

Benefited from the excellent optical characteristic, amorphous polymers (e.g., PMMA, PS, and PC) were regarded as ideal materials for manufacturing microfluidics and microoptical components. However, the applications of semicrystalline polymer microstructure products were rarely reported before.

As an important part of our research on functional microstructure devices, micro V-cuts on PP substrates can be used in metal-polymer composite heat exchanger. It comprised metal thermal conductive element on bottom and polymer thermal dissipation unit on top. PP substrates with microstructures on surface can increase the specific surface area of heat exchanger and enhance the thermal dissipation performance within limited space [40, 41]. Sample 7 with a diagnostic replication rate of 95.04% mentioned above was chosen as the polymer layer of this metal-polymer composite heat exchanger. Figure 12a showed the schematic views of metal-polymer composite heat exchanger with V-shape microgrooves and commercial aluminum heat exchanger with rectangular fins.

These two kinds of heat exchangers were placed on heat sources with same heating power, and the thermal dissipation performance tests were performed by comparing the real-time data of heat source temperature. It can be seen from Fig. 12b that the temperature of metal-polymer composite heat exchanger was a little lower than that of aluminum heat exchanger throughout the measuring process. Finally, the heat source temperature of aluminum heat exchanger and metal-polymer composite heat exchanger came into balance at 96.8[degrees]C and 94.4[degrees]C, respectively. It indicated that the thermal dissipation performances of them were almost the same. Considering the space occupied, it was obvious that our metal-polymer composite heat exchanger had much stronger thermal dissipation performances per unit volume than aluminum ones. The orientation and recrystallization of PP during P2P IHE process also guaranteed the geometry stability of polymer layer under high application temperature.

CONCLUSIONS

A new type of Plate to Plate isothermal hot embossing method (P2P IHE method) for semicrystalline polymers was proposed and discussed to simplify the hot embossing process in this paper. Utilizing the law of stress-strain behavior with temperature change, relatively short embossing cycle time of 20 s was achieved with constant and low mold temperature that 40[degrees]C to 60[degrees]C below [T.sub.m] in whole embossing cycle. The appropriate parameters for fabricating PP substrates with micro V-cuts were experimentally determined as 115[degrees]C of mold temperature, no <5 MPa of pressure and no <20 s of holding time. In sum, the experiments had proved the feasibility of microstructure fabrication with accurate replication and high processing efficiency on PP substrates by P2P IHE method. Excellent geometric stability of PP products fabricated by P2P IHE method was also confirmed by postprocessing under 110[degrees]C for 3 h. Furthermore, metal-polymer composite heat exchanger with perfect thermal dissipation performance was presented as an application example of PP microstructure product made by P2P IHE method. It is very critical to fabricate products with microstructure at vacuum state to ensure replication quality, ignoring relative acceptable replication quality of present nonvacuum embossed PP substrates benefited by its good gas discharging structure of V-cut. The rectangle microstructures with a fair aspect ratio and more other convincing structures will be used to further demonstrate the fabrication capability of this new P2P IHE method in the future.

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Sun Jingyao (iD), (1) Wu Darning, (1,2) Liu Ying, (2) Yang Zhenzhou, (1) Gou Pengsheng (1)

(1) Beijing University of Chemical Technology, Beijing, 100029, China

(2) State Key Laboratory of Organic-Inorganic Composites, Beijing, 100029, China

Correspondence to: L. Ying; e-mail: liuying@mail.buct.edu.cn Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 51673020; 51173015.

We hereby confirm that this manuscript is our original work and has not been published nor has it been submitted simultaneously elsewhere. We further confirm that all authors have checked the manuscript and have agreed to the submission.

DOI 10.1002/pen.24651

Caption: FIG. 1. Schematic view of the conventional micro hot embossing process (7r, room temperature; TE, embossing temperature; TD, demolding temperature; PE, embossing pressure).

Caption: FIG. 2. Molding windows of semicrystalline polymers. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. Procedure diagram of P2P isothermal hot embossing process for semicrystalline polymers ([T.sub.mold], mold temperature).

Caption: FIG. 4. WAXD curves of PP before and after embossed by P2P isothermal hot embossing method. [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 5. The stress-relaxation curves of PP at (a) 50[degrees]C, (b) 105[degrees]C, and (c) 115[degrees]C.

Caption: FIG. 6. Section view of the embossing mold.

Caption: FIG. 7. Flow chart of procedure process of P2P isothermal hot embossing.

Caption: FIG. 8. Cross profiles of the PP samples by P2P isothermal hot embossing method with a pressure of 10 MPa and holding time of 60 s: (a) Sample I: [T.sub.mold], 105[degrees]C; (b) Sample 2: [T.sub.mold], 110[degrees]C; (c) Sample 3: [T.sub.mold], 115[degrees]C; (d) Sample 4: [T.sub.mold], 125[degrees]C. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. Cross profiles of the PP samples by P2P isothermal hot embossing method with different pressure and holding time: (a) Sample 5: [T.sub.mold], 110[degrees]C; P, 6 MPa; t, 20 s; (b) Sample 6: [T.sub.mold], 110[degrees]C; P, 6 MPa; t, 20 s; (c) Sample 7: [T.sub.mold], 115[degrees]C; P, 6 MPa; t, 20 s; (d) Sample 8: [T.sub.mold], 115[degrees]C; P, 2 MPa; t, 20 s; (e) Sample 9: [T.sub.mold], 115[degrees]C; P, 5 MPa; r, 15 s; (f) Sample 10: [T.sub.mold], 115[degrees]C; P, 5 MPa; t, 30 s. [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 10. Overhead view of the PP sample with high replication precision. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 11. Cross profiles of the PP samples with and without postprocessing: (a) original; (b) reheated for 3 h. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 12. Comparison of metal-polymer composite heat exchanger and aluminum heat exchanger: (a) schematic views; (b) measuring results of heat source temperature. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Main properties of GEHR PP[R].

Properties                Test methods    Data        Unit

Density                   ISO 1183        0.91    g/[cm.sup.3]
Vicat softening point     ISO 306          91      [degrees]C
Distortion temperature    ISO 75           96      [degrees]C
Tensile yield strength    ISO 527          30          MPa

TABLE 2. Parameters of P2P isothermal hot embossing for PP with a
pressure of 10 MPa and holding time of 60 s.

Sample number    Mold temperature/[degrees]C
                 [T.sub.mold]

1                105
2                110
3                115
4                125

TABLE 3. Main results of P2P isothermal hot embossing for PP from
sample 1 to sample 4.

Sample
number       L ([micro]m)     H ([micro]m)    A ([degrees])

Sample 1          49               14         116.635
Sample 2          49               22         98.670
Sample 3          49               25         96.782
Sample 4          49               16         /
Mold        Lo = 49; H0 = 24.2; /lo = 90.727

                                Replication feature

Sample                                                    Diagnostic
number      L/[L.sub.0]    H/[H.sub.0]    A/[A.sub.0]    replication
             x 100 (%)      x 100 (%)      x 100 (%)       rate (%)

Sample 1        100           57.85          128.56         57.85
Sample 2        100           90.91          108.75         90.91
Sample 3        100           103.31         106.67         106.67
Sample 4        100           66.12            /            66.12
Mold

TABLE 4. Parameters of P2P isothermal hot embossing for PP.

Sample    Mold temperature     Pressure      Holding
number      ([degrees]C),      (MPa) P     time (s), t
            [T.sub.mold]

5                110              6             20
6                                 2
7                115              6
8                                 2
9                115              5             15
10                                              30

TABLE 5. Main results of the P2P isothermal hot embossing for PP from
sample 5 to sample 10.

Sample
number       L ([micro]m)   H ([micro]m)   A ([degrees])

Sample 5          49             20           101.600
Sample 6          49             9               /
Sample 7          49             23           93.386
Sample 8          49             11              /
Sample 9          49             18           109.094
Sample 10         49             23           99.170
Mold         [L.sub.0] = 49; [H.sub.0] = 24.2; [A.sub.0] = 90.727

                                 Replication feature

Sample                                                     Diagnostic
number       L/[L.sub.0]    H/[H.sub.0]    A/[A.sub.0]    replication
              x 100 (%)      x 100 (%)      x 100 (%)       rate (%)

Sample 5         100           82.64          111.98         82.64
Sample 6         100           37.19            /            37.19
Sample 7         100           95.04          102.93         95.04
Sample 8         100           45.45            /            45.45
Sample 9         100           74.38          120.24         74.38
Sample 10        100           95.04          109.31         109.31
Mold
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Author:Jingyao, Sun; Darning, Wu; Ying, Liu; Zhenzhou, Yang; Pengsheng, Gou
Publication:Polymer Engineering and Science
Article Type:Report
Date:Jun 1, 2018
Words:6495
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