*3.1. SEC Results*

The used PLA was modified on a twin-screw extruder with DCUP (0.1 wt%/0.2 wt%) and PDCI (0.5 wt%). After the reactive modification, the effectiveness of the modifiers was verified. One way to investigate this is by the method of SEC, which can be used to determine the molar mass and a molar mass distribution. The principle is based on the separation of molecules according to their molecular size. Figure 2 shows the molar mass and polydispersity index (PDI) of all produced compounds.

polydispersity index (PDI) of all produced compounds.

polydispersity index (PDI) of all produced compounds.

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**Figure 2.** SEC-Results of all PLA compounds. The organic peroxide led, as well as the PDCI, to an increased molar mass. As the amount of DCUP increases, this effect is enhanced. The addition of PCDI leads also to an increase in molar mass but is less pronounced than DCUP. The reason why the molar mass does not increase that much is that PCDI only reacts with the end groups of PLA. The combination of DCUP and PCDI showed the highest increase in molar mass. The results show on the one hand that a successful modification took place during the reactive extrusion on the twin-screw extruder and thus a chemical reaction between the two components has occurred. Furthermore, the increase in molar mass indicates a change in the linear structure of the PLA, which will be considered in more detail in the following rhe-The organic peroxide led, as well as the PDCI, to an increased molar mass. As the amount of DCUP increases, this effect is enhanced. The addition of PCDI leads also to an increase in molar mass but is less pronounced than DCUP. The reason why the molar mass does not increase that much is that PCDI only reacts with the end groups of PLA. The combination of DCUP and PCDI showed the highest increase in molar mass. The results show on the one hand that a successful modification took place during the reactive extrusion on the twin-screw extruder and thus a chemical reaction between the two components has occurred. Furthermore, the increase in molar mass indicates a change in the linear structure of the PLA, which will be considered in more detail in the following rheological investigations. amount of DCUP increases, this effect is enhanced. The addition of PCDI leads also to an increase in molar mass but is less pronounced than DCUP. The reason why the molar mass does not increase that much is that PCDI only reacts with the end groups of PLA. The combination of DCUP and PCDI showed the highest increase in molar mass. The results show on the one hand that a successful modification took place during the reactive extrusion on the twin-screw extruder and thus a chemical reaction between the two components has occurred. Furthermore, the increase in molar mass indicates a change in the linear structure of the PLA, which will be considered in more detail in the following rheological investigations. *3.2. Rheological Investigation* 

verified. One way to investigate this is by the method of SEC, which can be used to determine the molar mass and a molar mass distribution. The principle is based on the separation of molecules according to their molecular size. Figure 2 shows the molar mass and

verified. One way to investigate this is by the method of SEC, which can be used to determine the molar mass and a molar mass distribution. The principle is based on the separation of molecules according to their molecular size. Figure 2 shows the molar mass and

#### ological investigations. *3.2. Rheological Investigation* Figure 3 shows the measured frequency sweeps of the unmodified PLA compared to

*3.2. Rheological Investigation*  Figure 3 shows the measured frequency sweeps of the unmodified PLA compared to Figure 3 shows the measured frequency sweeps of the unmodified PLA compared to the modified ones. the modified ones.

**Figure 3. Figure 3.** Rheological measurements of the PLA compounds. Rheological measurements of the PLA compounds.

**Figure 3.** Rheological measurements of the PLA compounds. All materials show a typical shear thinning behavior. Pure, unmodified PLA shows a chain progression typical for linear polymers. Modification with DCUP leads to an increase in zero viscosity, but the curves still show typical shear thinning behavior and form a Newtonian plateau in the low-frequency range, except PLA with 0.2. wt% DCUP and 0.5 wt% PCDI. With higher amount of DCUP, a higher zero shear viscosity is observed. The increase in viscosity can be explained by the change in the molecular chain structure of the modified PLA. DCUP decomposes during reactive extrusion to form radicals that can attack the PLA chain leading to a chain extension or/and branching. It has already been

described in the literature that DCUP can lead to branching or cross-linking of polymer chains [12]. The longer chains and branching in the modified PLA result in entanglements that act like a physical network, leading to longer relaxation times. This leads to the fact that the Newtonian range is only indicated at low frequencies. The addition of 0.2 wt% DCUP is more effective than that of 0.1 wt% because a higher number of possible reactive groups are available which leads to an amount of extensions. PCDI was added as a hydrolysis stabilizer. As seen with the SEC results before, the addition of 0.5 wt% PCDI leads to an increase in molar mass und therefore also to an increase in the complex viscosity compared to neat PLA. Consequently, the combination of DCUP with PCDI results in the highest complex viscosity of all materials and to a change in the curve flow due to a complementation of the chain extension and the prevention to degradation. mer chains [12]. The longer chains and branching in the modified PLA result in entanglements that act like a physical network, leading to longer relaxation times. This leads to the fact that the Newtonian range is only indicated at low frequencies. The addition of 0.2 wt% DCUP is more effective than that of 0.1 wt% because a higher number of possible reactive groups are available which leads to an amount of extensions. PCDI was added as a hydrolysis stabilizer. As seen with the SEC results before, the addition of 0.5 wt% PCDI leads to an increase in molar mass und therefore also to an increase in the complex viscosity compared to neat PLA. Consequently, the combination of DCUP with PCDI results in the highest complex viscosity of all materials and to a change in the curve flow due to a complementation of the chain extension and the prevention to degradation. Timesweep measurements were used to observe the thermal stability. The thermal

All materials show a typical shear thinning behavior. Pure, unmodified PLA shows a chain progression typical for linear polymers. Modification with DCUP leads to an increase in zero viscosity, but the curves still show typical shear thinning behavior and form a Newtonian plateau in the low-frequency range, except PLA with 0.2. wt% DCUP and 0.5 wt% PCDI. With higher amount of DCUP, a higher zero shear viscosity is observed. The increase in viscosity can be explained by the change in the molecular chain structure of the modified PLA. DCUP decomposes during reactive extrusion to form radicals that can attack the PLA chain leading to a chain extension or/and branching. It has already been described in the literature that DCUP can lead to branching or cross-linking of poly-

Timesweep measurements were used to observe the thermal stability. The thermal stability is crucial for the processing, especially for the foaming process of polymers. In addition, information is obtained about the conversion of the chemical reaction during the reactive extrusion. Degradation reactions as well as post-reactions are generally undesirable and should be avoided or at least taken into account. The normalized complex viscosity as a function of time is shown in Figure 4. stability is crucial for the processing, especially for the foaming process of polymers. In addition, information is obtained about the conversion of the chemical reaction during the reactive extrusion. Degradation reactions as well as post-reactions are generally undesirable and should be avoided or at least taken into account. The normalized complex viscosity as a function of time is shown in Figure 4.

*Polymers* **2021**, *13*, x FOR PEER REVIEW 5 of 7

**Figure 4.** Normalized time-dependent viscosity of the compounds. **Figure 4.** Normalized time-dependent viscosity of the compounds.

The time sweep was done for 30 min at isothermal temperature of 180 °C for all investigated compounds. It can be seen that the neat PLA has the highest degree of degradation, followed by the different peroxide-containing compounds. The PCDI stabilized material and the modified stabilized modifications show the highest relative viscosity indicating no or even a low degradation during the timesweep. After the first 400 s, for all materials except those with PCDI, the normalized complex viscosity decreases due to degradation processes. The use of PCDI led to the desired stabilization process. However, only a slight decrease in complex viscosity for the PLA with 0.5 wt% PCDI can be observed after 30 min. Additionally, the reaction regarding the chain extension proceeded completely and no post-reactions could be observed. The combination of DCUP and PCDI has The time sweep was done for 30 min at isothermal temperature of 180 ◦C for all investigated compounds. It can be seen that the neat PLA has the highest degree of degradation, followed by the different peroxide-containing compounds. The PCDI stabilized material and the modified stabilized modifications show the highest relative viscosity indicating no or even a low degradation during the timesweep. After the first 400 s, for all materials except those with PCDI, the normalized complex viscosity decreases due to degradation processes. The use of PCDI led to the desired stabilization process. However, only a slight decrease in complex viscosity for the PLA with 0.5 wt% PCDI can be observed after 30 min. Additionally, the reaction regarding the chain extension proceeded completely and no post-reactions could be observed. The combination of DCUP and PCDI has the highest complex viscosity and is stable over the time of 1800 s.
