3.2. Damping Properties of the Latex Film
In this study, the damping properties of the samples were measured by DMA.
The temperature–tan δ curves of CS1, CS2, CST1, and CST2 at 1 Hz are shown in
Figure 3. The temperature–tan δ curves of CS3, CS4, CST3, and CST4 at 1 Hz are shown in
Figure 4.
Table 4 presents the
Tg of each layer for all samples based on the Fox equation and the damping parameters of samples. In the following,
Tg stands for the temperature of the loss peaks. The theoretical
Tg was calculated by FOX.
As can be seen from
Figure 3, two distinct characteristic peaks appear on the CS1 curve, in which 100.84 °C is attributed to the core polymer of the latex particle, and 6.55 °C is attributed to the shell polymer. The theoretical
Tg of the core and the shell polymer of CS1 are 96.07 °C and −13.31 °C, respectively. Similarly, CS2 also shows a similar curve to CS1, while both peaks (74.83 °C and 16.84 °C) are closer to each other, and the curve between both peaks show a platform structure. On the curves of CST1 and CST2, the same phenomenon occurs.
It is known by reference [
29] that the
Tg obtained from the temperature–tan δ curve is higher than that of other test methods such as DSC, TMA, etc. The tan δ peak of the high-temperature part has shifted to the left, and the low-temperature part has shifted to the right, and the two peaks are distinct. Obviously, the phase separation happened in CS1 and CST1.
In
Figure 3, it can be found that the two tan δ peak values of CST1 that contain the “transition layer” have decreased slightly, whereas the corresponding
Tg remains almost unchanged. This may be due to the fact that the composition of the core polymer is the same, and also the
Tg of the core part is higher than the reaction temperature, and it is in a glassy transition state during the subsequent reaction. It also can be found that the
Tg at the low temperature of CST1 shifted to the right, the tan δ peak value decreased, and the transition between both peaks became smoother. Two effects may have caused the phenomenon. Firstly, by comparing with CS1, the addition of the “transition layer” MBA of CST1 resulted in the
Tg reduction of the shell layer. Secondly, after blending the shell of CST1 with the “transition layer”, PMBA causes the
Tg to shift to the right. A similar phenomenon can be seen in the comparison of CS2 and CST2; only the changes between the two peaks of the CS2 and CST2 curves are smoother. It is worth noting that CST2 combines the core layer and the shell layer due to the “bridge” functions as the “transition layer”, which can be interpreted as forming a latex interpenetrating polymer network (LIPN) structure [
6,
20,
21].
From
Figure 4, it can be observed that as the
Tg of the core polymer decreases, the total number of materials remains unchanged. In other words, the
Tg of the shell part also increases accordingly, and the difference at
Tg between the core and the shell is narrowing; thus, only one peak is observed on the curve. This indicates that unlike CS1 and CS2, phase separation has not occurred in CS3 and CS4, and better compatibilities appear in the core and shell. The same applies to the curves of CST3 and CST4.
The temperature–tan δ curves of CS3, CST3, CS4, and CST4 are all single peaks, which indicates that the shell polymer is more easily compatible with the core polymer without phase separation. By further comparison, the Tg of CST3 is higher than that of CS3, and such cases also occurred in CST4 and CS4. It might be explained that compared with the core–shell structure without the “transition layer”, the “transition layer” is a homopolymer composed of a single monomer with moderate Tg, and the solubility parameter is closer to the core and shell polymers. Therefore, physical entanglement is more likely to occur in macromolecules, making the intramolecular motion more obstructed, and then it shifts the tan δ peak to the right.
There are two possible reasons for the single peak. Firstly, as the difference at Tg between the core and shell gets smaller (from 109.38 °C of CS1 to 16.04 °C of CS4), the compatibility between the core and shell improved. In addition, it may also be because the reaction temperature is 75–85 °C. During the reaction process, CS3 and CS4 were in a viscous flow state, the monomer and the core were more compatible, and the boundary between the two phases becomes more blurred due to the phase continuity formed, which shows a single peak on the temperature–tan δ curve.
Reflecting on the damping performance, due to more obstacles in molecular motion, there is both an increase in the physical cross-linking network and internal friction between molecules. Compared with CS3 and CS4, the tan δmax of CST3 and CST4 respectively increased by 15.49% and 29.11%. However, the effective damping temperature (tan δ > 0.3) range did not significantly improve compared to CS4 and CST4; this might be because the chain of the “transition layer” does not significantly affect the internal friction in low and high temperature (glassy state and viscous state).
3.3. Activation Ennergy of Latex in Glass Trasition
Figure 5 shows the temperature–tan δ curves of latexes in 1, 10, 25, 50, and 100 Hz. It is well known that the dynamic mechanical behavior of viscoelastic damping materials is directly related to time (deformation frequency) and temperature. It can easily be seen that when the frequency increases from
Figure 5, the curve moves toward the high-temperature direction. This is consistent with Time–Temperature Superposition (TTS).
According to the relationship between the experimental frequency ω and the obtained transition temperature
T (absolute temperature, K),
In the formula, Δ
E is the activation energy of the corresponding moving unit, J/mol,
R = 8.314 J/mol/°C, which is a constant. ω is the measurement frequency, Hz. Take the logarithm of both sides of the above formula to get [
30,
31]:
By making the curve (linear fitting) of 1/
Tg–ln ω, the slope of the curve obtained is—Δ
E/
R, which is shown in
Figure 6.
Table 5 shows the corresponding
Tg of latex particles at different frequencies (the corresponding temperature of the peaks).
Figure 6 shows the curve of 1/
Tg–ln ω. By comparing the activation energy of CS1, CS2, CS3, and CS4, CST1, CST2, CST3, and CST4, it was found that as the
Tg between the core and shell becomes closer, the activation energy required for the latex particles to undergo glass transition increases. This is consistent with the aforementioned
Tg of CS4 being higher than that of CS3.
By comparing the series of CS and CST, it can be found that the
Tg activation energy of CS1 and CS2 is higher than that of CST1 and CST2, respectively. This means that it is harder for CS1 and CS2 to transition to the rubbery state. It could be explained by the introduction of the “transition layer” making the chains of polymers easier to move and reducing the difference at
Tg between the core and shell. On the contrary, the
Tg activation energy of CST3 and CST4 is higher than that of CS3 and CS4, respectively. This indicates that the chains of CST3 and CST4 need more energy to move in the glass transition, and this is consistent with the aforementioned
Tg of CST4 being higher than that of CS4, which has been explained in
Section 3.2.
3.4. Tensile Strength of Latex Film
The stress–strain curves of the latex film are shown in
Figure 7, and
Table 6 shows the tensile strength and elongation at break of latex film. It is found that with the decrease of the
Tg difference at
Tg between the core and shell, the tensile properties of the latex films increase first and then decrease, but the elongation at break keeps decreasing. The introduction of the “transition layer” does not change this trend.
There are two possible reasons for this. On the one hand, as the core Tg decreases, the amount of St in the shell polymer increases, and the amount of soft monomer decreases, which results in the hardening of the shell polymer. At this level, the tensile strength of the polymer is improved. On the other hand, as the Tg of the core polymer decreases, the core polymer becomes soft, and the soft core has a weakening effect on the mechanical properties of the latex film. Under the combined action of the above two, the tensile strength of the latex film increases firstly and then decreases. In addition, it could be obtained a latex film with the best mechanical properties by adjusting the Tg difference between the core and shell.
From
Figure 7, it can also be found that the tensile strength of the latex film with the “transition layer” structure is improved to different degrees than the latex film without the “transition layer” structure, 20.53%, 17.09%, 24.61%, and 36.73% respectively. This indicates that the introduction of the “transition layer” can effectively improve the tensile strength of the latex film under the same formulation. This may be because the introduction of the “transition layer” makes the connection between the core and the shell closer, and the interface between the core and the shell is stronger. This can also be confirmed from the comparison of the activation energy in
Table 5. Compared with the CS series without a “transition layer”, the elongation at break of the CST series latex films is reduced. That can be interpreted as the introduction of the “transition layer” having a positive effect on the compatibility between the core and shell of the latex films. In addition, compared with CS, the proportion of ST in the outermost layer of CST is higher, which limits the inter–chain motion of some molecules, leading to the decrease of elongation at break.
CST3 exhibits an obvious yielding process, and CS3 also exhibits a yielding process from the stress–strain curves in
Figure 7. To further analyze the reasons for the significant difference in the shape of tensile stress–strain curves, SEM was used to observe the cross-sectional images of the tensile test specimens. As shown in
Figure 8, CST3 and CS3 exhibit a much more rough with shear deformation. Shear bands with higher roughness lead to longer crack propagation paths, which prevents deformation and crack growth and results in higher mechanical strength values. Compare CS1 and CST1 as well as CS3 and CST3. For the cross-sectional images of the tensile test specimen latex film, the “transition layer” also exhibits a relatively rough surface, which results in higher mechanical strength values.